<PAGE>

                                                                   EXHIBIT 10.12


                               ALTAVISTA COMPANY

                           INVESTOR RIGHTS AGREEMENT

     This Agreement dated as of March __, 2000 is entered into by and among
AltaVista Company, a Delaware corporation (the "Company"), and CMGI, Inc., a
Delaware corporation (the "Investor").

                                   Recitals
                                   --------

     WHEREAS, the Company desires to undertake an initial public offering of its
Common Stock; and

     WHEREAS, in order to induce the Investor to approve such offering, the
Company has agreed to provide for certain arrangements with respect to (i) the
registration of shares of capital stock of the Company under the Securities Act
of 1933, as amended, and (ii) the Investor's right of first refusal with respect
to certain issuances of securities of the Company;

     NOW, THEREFORE, in consideration of the mutual promises and covenants
contained in this Agreement, the parties hereto agree as follows:

     I.   Certain Definitions.
          -------------------

     As used in this Agreement, the following terms shall have the following
respective meanings:

     "Commission" means the Securities and Exchange Commission, or any other
      ----------
federal agency at the time administering the Securities Act.

     "Common Stock" means the common stock, $.01 par value per share, of the
      ------------
Company.

     "Exchange Act" means the Securities Exchange Act of 1934, as amended, or
      ------------
any successor federal statute, and the rules and regulations of the Commission
issued under such Act, as they each may, from time to time, be in effect.

     "Initiating Holders" means the Stockholders initiating a request for
      ------------------
registration pursuant to Section 2.1(a) or 2.1(b), as the case may be.
<PAGE>

     "Initial Public Offering" means the initial underwritten public offering of
      -----------------------
shares of Common Stock pursuant to an effective Registration Statement.

     "Permitted Transferee" shall have the meaning set forth in Section 3.3.
      --------------------

     "Prospectus" means the prospectus included in any Registration Statement,
      ----------
as amended or supplemented by an amendment or prospectus supplement, including
post-effective amendments, and all material incorporated by reference or deemed
to be incorporated by reference in such Prospectus.

     "Registration Statement" means a registration statement filed by the
      ----------------------
Company with the Commission for a public offering and sale of securities of the
Company (other than a registration statement on Form S-8 or Form S-4, or their
successors, or any other form for a similar limited purpose, or any registration
statement covering only securities proposed to be issued in exchange for
securities or assets of another corporation).

     "Registration Expenses" means the expenses described in Section 2.4.
      ---------------------

     "Registrable Shares" means (a) the shares of Common Stock held by the
      ------------------
Investor upon the closing of the Initial Public Offering and (b) any other
shares of Common Stock issued in respect of such shares (because of stock
splits, stock dividends, reclassifications, recapitalizations or similar
events); provided, however, that shares of Common Stock which are Registrable
         --------  -------
Shares shall cease to be Registrable Shares upon (i) any sale pursuant to a
Registration Statement or Rule 144 under the Securities Act or (ii) any sale in
any manner to a person or entity which, by virtue of Section 3.4 of this
Agreement, is not entitled to the rights provided by this Agreement.

     "Securities Act" means the Securities Act of 1933, as amended, or any
      --------------
successor federal statute, and the rules and regulations of the Commission
issued under such Act, as they each may, from time to time, be in effect.

     "Selling Stockholder" means any Stockholder owning Registrable Shares
      -------------------
included in a Registration Statement.

     "Stockholders" means the Investor and any persons or entities to whom the
      ------------
rights granted under this Agreement are transferred by the Investor, its
successors or assigns, pursuant to Section 3.4 hereof.


                                       2
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     II.  Registration Rights.
          -------------------

          2.1  Required Registrations.
               ----------------------

               (a)  At any time following 180 days after the closing of the
Initial Public Offering, a Stockholder or Stockholders may request, in writing,
that the Company effect the registration on Form S-1 or Form S-2 (or any
successor form) of Registrable Shares owned by such Stockholder or Stockholders
having an aggregate value of at least $10,000,000 (based on the then current
public market price).

               (b)  At any time after the Company becomes eligible to file a
Registration Statement on Form S-3 (or any successor form relating to secondary
offerings), a Stockholder or Stockholders may request, in writing, that the
Company effect the registration on Form S-3 (or such successor form), of
Registrable Shares having an aggregate value of at least $2,500,000 (based on
the then current public market price).

               (c)  Upon receipt of any request for registration pursuant to
this Section 2.1, the Company shall promptly give written notice of such
proposed registration to all other Stockholders. Such Stockholders shall have
the right, by giving written notice to the Company within 15 days after the
Company provides its notice, to elect to have included in such registration such
of their Registrable Shares as such Stockholders may request in such notice of
election, subject in the case of an underwritten offering to the approval of the
managing underwriter as provided in Section 2.1(d) below. Thereupon, the Company
shall, as expeditiously as possible, use its best efforts to effect the
registration on an appropriate registration form of all Registrable Shares which
the Company has been requested to so register (provided, however, that in the
                                               --------  -------
case of a registration requested under Section 2.1(b), the Company will only be
obligated to effect such registration on Form S-3 (or any successor form)).

               (d)  If the Initiating Holders intend to distribute the
Registrable Shares covered by their request by means of an underwriting, they
shall so advise the Company as a part of their request made pursuant to Section
2.1(a) or (b), as the case may be, and the Company shall include such
information in its written notice referred to in Section 2.1(c). The right of
any other Stockholder to include its Registrable Shares in such registration
pursuant to Section 2.1(a) or (b), as the case may be, shall be conditioned upon
such other Stockholder's participation in such underwriting on the terms set
forth herein. If the managing underwriter determines that the marketing factors

                                       3
<PAGE>

require a limitation of the number of shares to be underwritten, the number of
Registrable Shares to be included in a Registration Statement filed pursuant to
this Section 2.1 shall be reduced pro rata among the requesting Stockholders
based on the quotient of (i) the total Registrable Shares to be included in the
Registration Statement, divided by (ii) the total number of Registrable Shares
that requested registration.

          (e)  The Initiating Holders shall have the right to select the
managing underwriter(s) for any underwritten offering requested pursuant to
Section 2.1(a) or (b), subject to the approval of the Company, which approval
will not be unreasonably withheld.

          (f)  The Company shall not be required to effect more than two
registrations pursuant to Section 2.1(a) or more than five registrations
pursuant to Section 2.1(b).  In addition, the Company shall not be required to
effect any registration within 90 days after the effective date of any other
Registration Statement of the Company relating to an underwritten offering.  For
purposes of this Section 2.1(f), a Registration Statement shall not be counted
until such time as such Registration Statement has been declared effective by
the Commission, unless the Initiating Holders withdraw their request for such
registration (other than as a result of information concerning the business or
financial condition of the Company which is made known to the Stockholders after
the date on which such registration was requested) and elect not to pay the
Registration Expenses therefor pursuant to Section 2.4.

          (g)  If at the time of any request to register Registrable Shares by
Initiating Holders pursuant to this Section 2.1, the Company is engaged or has
plans to engage in a registered public offering or is engaged in any other
activity which, in the good faith determination of the Company's Board of
Directors, would be adversely affected by the requested registration or if
financial statements required for the requested registration are not then
available, then the Company may at its option direct that such request be
delayed for a period not in excess of 90 days from the date of such request,
such right to delay a request to be exercised by the Company not more than once
in any 12-month period.

     2.2  Incidental Registration.
          -----------------------

          (a)  Whenever the Company proposes to file a Registration Statement
(other than a Registration Statement filed pursuant to Section 2.1) at any time
and from time to time, it will, prior to such filing, give written notice to all
Stockholders of its intention to do so; provided, that no such notice need be
                                        --------
given if no Registrable

                                       4
<PAGE>

Shares are to be included therein as a result of a determination of the managing
underwriter pursuant to Section 2.2(b). Upon the written request of a
Stockholder or Stockholders given within 20 days after the Company provides such
notice (which request shall state the intended method of disposition of such
Registrable Shares), the Company shall use its best efforts to cause all
Registrable Shares which the Company has been requested by such Stockholder or
Stockholders to register to be registered under the Securities Act to the extent
necessary to permit their sale or other disposition in accordance with the
intended methods of distribution specified in the request of such Stockholder or
Stockholders; provided, that the Company shall have the right to postpone or
              --------
withdraw any registration effected pursuant to this Section 2.2 without
obligation to any Stockholder.

          (b)  If the registration for which the Company gives notice pursuant
to Section 2.2(a) involves an underwriting, the Company shall so advise the
Stockholders as a part of the written notice given pursuant to Section 2.2(a).
In such event, the right of any Stockholder to include its Registrable Shares in
such registration pursuant to Section 2.2 shall be conditioned upon such
Stockholder's participation in such underwriting on the terms set forth herein.
All Stockholders proposing to distribute their securities through such
underwriting shall enter into an underwriting agreement in customary form with
the underwriter or underwriters selected for the underwriting by the Company.
Notwithstanding any other provision of this Agreement, if the Company and the
managing underwriter(s) determine in good faith that marketing factors require a
limitation of the number of shares to be underwritten, then the Company and the
managing underwriter(s) may exclude shares from the registration and the
underwriting, and the number of shares that may be included in the registration
and the underwriting shall be allocated, first to the Company or the Company
                                         -----
stockholder(s) for which the registration was initiated, and second to each of
                                                             ------
the Stockholders requesting inclusion of their Registrable Shares in such
registration and each of the other holders of piggyback registration rights on a
parity with those Stockholders on a pro rata basis based on the total number of
Registrable Shares and other securities requested for inclusion in such
registration by each such Stockholder or other holder. If any holder of
Registrable Shares or any other Company stockholder requesting inclusions of
securities in the registration disapproves of the terms of any such
underwriting, such person may elect to withdraw therefrom by written notice to
the Company, and any Registrable Shares or other securities excluded or
withdrawn from such underwriting shall be withdrawn from such registration.

               (c)  Notwithstanding the foregoing, the Company shall not be
required, pursuant to this Section 2.2, to include any Registrable Shares in a
Registration

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<PAGE>

Statement if such Registrable Shares can then be sold pursuant to Rule 144(k)
under the Securities Act and represent less than 1% of the then outstanding
shares of Common Stock.

          2.3  Registration Procedures.
               -----------------------

               (a)  If and whenever the Company is required by the provisions of
this Agreement to use its best efforts to effect the registration of any
Registrable Shares under the Securities Act, the Company shall:

                    (i)   file with the Commission a Registration Statement with
     respect to such Registrable Shares and use its best efforts to cause that
     Registration Statement to become effective as soon as possible;

                    (ii)  as expeditiously as possible, prepare and file with
     the Commission any amendments and supplements to the Registration Statement
     and the prospectus included in the Registration Statement as may be
     necessary to comply with the provisions of the Securities Act (including
     the anti-fraud provisions thereof) and to keep the Registration Statement
     effective for 12 months from the effective date or such lesser period until
     all such Registrable Shares are sold;

                    (iii) as expeditiously as possible, furnish to each Selling
     Stockholder such reasonable numbers of copies of the Prospectus, including
     any preliminary Prospectus, in conformity with the requirements of the
     Securities Act, and such other documents as such Selling Stockholder may
     reasonably request in order to facilitate the public sale or other
     disposition of the Registrable Shares owned by such Selling Stockholder;

                    (iv)  as expeditiously as possible, use its best efforts to
     register or qualify the Registrable Shares covered by the Registration
     Statement under the securities or Blue Sky laws of such states as the
     Selling Stockholders shall reasonably request and do any and all other acts
     and things that may be necessary or desirable to enable the Selling
     Stockholders to consummate the public sale or other disposition in such
     states of the Registrable Shares owned by the Selling Stockholder;
     provided, however, that the Company shall not be required
     --------  -------

                                       6
<PAGE>

     in connection with this paragraph (iv) to qualify as a foreign corporation
     or execute a general consent to service of process in any jurisdiction;

                    (v) as expeditiously as possible, cause all such Registrable
     Shares to be listed on each securities exchange or automated quotation
     system on which similar securities issued by the Company are then listed;
     and

                    (vi) promptly make available for inspection by the Selling
     Stockholders, any managing underwriter participating in any disposition
     pursuant to such Registration Statement and any attorney or accountant or
     other agent retained by any such underwriter or selected by the Selling
     Stockholders, all financial and other records, pertinent corporate
     documents and properties of the Company and cause the Company's officers,
     directors, employees and independent accountants to supply all information
     reasonably requested by any such Selling Stockholder, underwriter,
     attorney, accountant or agent in connection with such Registration
     Statement.

          (b)  If the Company has delivered a Prospectus to the Selling
Stockholders, and after having done so, the Prospectus is amended to comply with
the requirements of the Securities Act, the Company shall promptly notify the
Selling Stockholders and, if requested, the Selling Stockholders shall
immediately cease making offers of Registrable Shares and return all
Prospectuses to the Company. The Company shall promptly provide the Selling
Stockholders with revised Prospectuses, and following receipt of the revised
Prospectuses, the Selling Stockholders shall be free to resume making offers of
the Registrable Shares.

          (c) In the event that, in the judgment of the Company, it is advisable
to suspend use of a Prospectus included in a Registration Statement due to
pending material developments or other events that have not yet been publicly
disclosed and as to which the Company believes public disclosure would be
detrimental to the Company, the Company shall notify all Selling Stockholders to
such effect, and upon receipt of such notice, each such Selling Stockholder
shall immediately discontinue any sales of Registrable Shares pursuant to such
Registration Statement until such Selling Stockholder has received copies of a
supplemented or amended Prospectus or until such Selling Stockholder is advised
in writing by the Company that the then current Prospectus may be used and has
received copies of any additional or supplemental filings that are incorporated
or deemed incorporated by reference in such Prospectus.

                                       7
<PAGE>

Notwithstanding anything to the contrary herein, the Company shall not exercise
its rights under this Section 2.3(c) to suspend sales of Registrable Shares for
a period in excess of 90 days in any 365-day period.

          2.4  Allocation of Expenses.  The Company will pay all Registration
               ----------------------
Expenses for all registrations under this Agreement; provided, however, that if
                                                     --------  -------
a registration under Section 2.1 is withdrawn at the request of the Initiating
Holders (other than as a result of information concerning the business or
financial condition of the Company which is made known to the Stockholders after
the date on which such registration was requested) and if the Initiating Holders
elect not to have such registration counted as a registration requested under
Section 2.1, the requesting Stockholders shall pay the Registration Expenses of
such registration pro rata in accordance with the number of their Registrable
Shares included in such registration. For purposes of this Section, the term
"Registration Expenses" shall mean all expenses incurred by the Company in
complying with this Agreement, including, without limitation, all registration
and filing fees, exchange listing fees, printing expenses, fees and expenses of
counsel for the Company and the fees and expenses of one counsel selected by the
Selling Stockholders to represent the Selling Stockholders, state Blue Sky fees
and expenses and the expense of any special audits incident to or required by
any such registration, but excluding underwriting discounts, selling commissions
and the fees and expenses of Selling Stockholders' own counsel (other than the
counsel selected to represent all Selling Stockholders).

     2.5  Indemnification and Contribution.

          (a)  In the event of any registration of any of the Registrable Shares
under the Securities Act pursuant to this Agreement, the Company will indemnify
and hold harmless the Selling Stockholder, each underwriter of such Registrable
Shares and each other person, if any, who controls such Selling Stockholder or
underwriter within the meaning of the Securities Act or the Exchange Act against
any losses, claims, damages or liabilities, joint or several, to which such
Selling Stockholder, underwriter or controlling person may become subject under
the Securities Act, the Exchange Act, state securities or Blue Sky laws or
otherwise, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) (i) arise out of or are based upon any untrue statement or
alleged untrue statement of any material fact contained in any Registration
Statement under which such Registrable Shares were registered under the
Securities Act, any preliminary prospectus or final prospectus contained in the
Registration Statement or any amendment or supplement to such Registration
Statement or (ii) arise out of or are based upon the omission or alleged
omission to state a material fact required to be stated

                                       8
<PAGE>

therein or necessary to make the statements therein not misleading; and the
Company will reimburse such Selling Stockholder, underwriter and controlling
person for any legal or any other expenses reasonably incurred by such Selling
Stockholder, underwriter or controlling person in connection with investigating
or defending any such loss, claim, damage, liability or action; provided,
                                                                --------
however, that the Company will not be liable in any such case to the extent that
-------
any such loss, claim, damage or liability arises out of or is based upon any
untrue statement or omission made in such Registration Statement, preliminary
prospectus or prospectus, or any such amendment or supplement, in reliance upon
and in conformity with information furnished to the Company, in writing, by or
on behalf of such Selling Stockholder, underwriter or controlling person
specifically for use in the preparation thereof.

          (b)  In the event of any registration of any of the Registrable Shares
under the Securities Act pursuant to this Agreement, each Selling Stockholder,
severally and not jointly, will indemnify and hold harmless the Company, each of
its directors and officers and each underwriter (if any) and each person, if
any, who controls the Company or any such underwriter within the meaning of the
Securities Act or the Exchange Act, against any losses, claims, damages or
liabilities, joint or several, to which the Company, such directors and
officers, underwriter or controlling person may become subject under the
Securities Act, Exchange Act, state securities or Blue Sky laws or otherwise,
insofar as such losses, claims, damages or liabilities (or actions in respect
thereof) (i) arise out of or are based upon any untrue statement or alleged
untrue statement of a material fact contained in any Registration Statement
under which such Registrable Shares were registered under the Securities Act,
any preliminary prospectus or final prospectus contained in the Registration
Statement or any amendment or supplement to the Registration Statement or (ii)
arise out of or are based upon any omission or alleged omission to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading, if the statement or omission was made in reliance upon
and in conformity with information relating to such Selling Stockholder
furnished in writing to the Company by or on behalf of such Selling Stockholder
specifically for use in connection with the preparation of such Registration
Statement, prospectus, amendment or supplement; provided, however, that the
                                                --------  -------
obligations of a Selling Stockholder hereunder shall be limited to an amount
equal to the net proceeds to such Selling Stockholder of Registrable Shares sold
in connection with such registration.

          (c)  Each party entitled to indemnification under this Section 2.5
(the "Indemnified Party") shall give notice to the party required to provide
indemnification (the "Indemnifying Party") promptly after such Indemnified Party
has

                                       9
<PAGE>

actual knowledge of any claim as to which indemnity may be sought and shall
permit the Indemnifying Party to assume the defense of any such claim or any
litigation resulting therefrom; provided, that counsel for the Indemnifying
                                --------
Party, who shall conduct the defense of such claim or litigation, shall be
approved by the Indemnified Party (whose approval shall not be unreasonably
withheld); and, provided, further, that the failure of any Indemnified Party to
                --------  -------
give notice as provided herein shall not relieve the Indemnifying Party of its
obligations under this Section 2.5 except to the extent that the Indemnifying
Party is adversely affected by such failure. The Indemnified Party may
participate in such defense at such Indemnified Party's expense; provided,
                                                                 --------
however, that the Indemnifying Party shall pay such expense if representation of
-------
such Indemnified Party by the counsel retained by the Indemnifying Party would
be inappropriate due to actual or potential differing interests between the
Indemnified Party and any other party represented by such counsel in such
proceeding; provided further that in no event shall the Indemnifying Party be
            -------- -------
required to pay the expenses of more than one law firm per jurisdiction as
counsel for the Indemnified Party.  The Indemnifying Party also shall be
responsible for the expenses of such defense if the Indemnifying Party does not
elect to assume such defense.  No Indemnifying Party, in the defense of any such
claim or litigation shall, except with the consent of each Indemnified Party,
consent to entry of any judgment or enter into any settlement which does not
include as an unconditional term thereof the giving by the claimant or plaintiff
to such Indemnified Party of a release from all liability in respect of such
claim or litigation, and no Indemnified Party shall consent to entry of any
judgment or settle such claim or litigation without the prior written consent of
the Indemnifying Party, which consent shall not be unreasonably withheld.

          (d)  In order to provide for just and equitable contribution in
circumstances in which the indemnification provided for in this Section 2.5 is
due in accordance with its terms but for any reason is held to be unavailable to
an Indemnified Party in respect to any losses, claims, damages and liabilities
referred to herein, then the Indemnifying Party shall, in lieu of indemnifying
such Indemnified Party, contribute to the amount paid or payable by such
Indemnified Party as a result of such losses, claims, damages or liabilities to
which such party may be subject in such proportion as is appropriate to reflect
the relative fault of the Company on the one hand and the Selling Stockholders
on the other in connection with the statements or omissions which resulted in
such losses, claims, damages or liabilities, as well as any other relevant
equitable considerations. The relative fault of the Company and the Selling
Stockholders shall be determined by reference to, among other things, whether
the untrue or alleged untrue statement of material fact related to information
supplied by the Company or the Selling Stockholders and the parties' relative
intent, knowledge, access to information and

                                       10
<PAGE>

opportunity to correct or prevent such statement or omission. The Company and
the Stockholders agree that it would not be just and equitable if contribution
pursuant to this Section 2.5 were determined by pro rata allocation or by any
other method of allocation which does not take account of the equitable
considerations referred to above. Notwithstanding the provisions of this Section
2.5(d), (a) in no case shall any one Selling Stockholder be liable or
responsible for any amount in excess of the net proceeds received by such
Selling Stockholder from the offering of Registrable Shares and (b) the Company
shall be liable and responsible for any amount in excess of such proceeds;
provided, however, that no person guilty of fraudulent misrepresentation (within
--------  -------
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation. Any party entitled to contribution will, promptly after
receipt of notice of commencement of any action, suit or proceeding against such
party in respect of which a claim for contribution may be made against another
party or parties under this Section 2.5(d), notify such party or parties from
whom contribution may be sought, but the omission so to notify such party or
parties from whom contribution may be sought shall not relieve such party from
any other obligation it may have thereunder or otherwise under this Section
2.5(d). No party shall be liable for contribution with respect to any action,
suit, proceeding or claim settled without its prior written consent, which
consent shall not be unreasonably withheld.

          2.6  Other Matters with Respect to Underwritten Offerings.  In the
               ----------------------------------------------------
event that Registrable Shares are sold pursuant to a Registration Statement in
an underwritten offering pursuant to Section 2.1, the Company agrees to enter
into an underwriting agreement containing customary representations and
warranties with respect to the business and operations of the Company and
customary covenants and agreements to be performed by the Company, including
without limitation customary provisions with respect to indemnification by the
Company of the underwriters of such offering.

          2.7  Information by Holder.  Each Selling Stockholder shall furnish to
               ---------------------
the Company such information regarding such Selling Stockholder and the
distribution proposed by such Selling Stockholder as the Company may reasonably
request in writing and as shall be required in connection with any registration,
qualification or compliance referred to in this Agreement.


     III. Right Of First Refusal.
          ----------------------

                                       11
<PAGE>

          3.1  Rights of Investor.
               ------------------

               (a)  Until the first date on which the Investor or any Permitted
Transferee (as defined below) owns less than a majority, by voting power, of the
outstanding shares of capital stock of the Company, the Company shall not issue
or sell (i) any shares of its Common Stock, (ii) any other voting equity
securities of the Company, including, without limitation, shares of preferred
stock, (iii) any option, warrant or other right to subscribe for, purchase or
otherwise acquire any voting equity securities of the Company or (iv) any debt
securities convertible into voting capital stock of the Company (collectively,
the "Offered Securities"), unless in each such case the Company shall have first
complied with this Section 3.1.  The Company shall deliver to the Investor a
written notice of any proposed or intended issuance or sale of Offered
Securities (the "Offer"), which Offer shall (A) identify and describe the
Offered Securities, (B) describe the price and other terms upon which they are
to be issued or sold, and the number or amount of the Offered Securities to be
issued or sold, (C) identify the persons or entities (if known) to which or with
which the Offered Securities are to be offered, issued or sold and (D) offer to
issue and sell to the Investor a number of the Offered Securities (the
"Available Amount") such that, after the issuance and sale of all of the Offered
Securities, including the purchase of the Available Amount by the Investor, the
Investor would own at least a majority, by voting power, of the outstanding
capital stock of the Company (assuming the exercise and conversion of all
outstanding options, warrants and convertible securities).  The Company shall
not be required to offer any Offered Securities to the Investor hereunder if,
after the issuance and sale thereof, the Investor (or the Permitted Transferee)
would continue to own at least a majority, by voting power, of the outstanding
capital stock of the Company.

               (b)  To accept an Offer, in whole or in part, the Investor must
deliver a written notice to the Company within 20 days after its receipt of the
Offer, setting forth the portion of the Available Amount that the Investor
elects to purchase (the "Notice of Acceptance").

               (c)  The Company shall have 180 days from the expiration of the
period set forth in Section 3.1(b) above to issue or sell all or any part of
such Offered Securities as to which a Notice of Acceptance has not been given by
the Investor, upon terms and conditions which are not more favorable, in the
aggregate, to the acquiring person or persons or less favorable to the Company
than those set forth in the Offer. If the consideration to be received by the
Company from the sale of Offered Securities consists of anything other than
cash, the Board of Directors of the Company shall in good faith determine the
cash equivalent of such non-cash consideration and the Investor may

                                       12
<PAGE>

pay an equivalent portion of its purchase price for the elected portion of the
Available Amount in cash.

               (d) The purchase by the Investor of any Offered Securities is
subject in all cases to the preparation, execution and delivery by the Company
and the Investor of a purchase agreement relating to such Offered Securities
reasonably satisfactory in form and substance to the Investor.

               (e) The rights of the Investor under this Section 3.1 shall not
apply to the grant of options to officers, directors, consultants and employees
of the Company or any subsidiary pursuant to any plan, agreement or arrangement
approved by a vote of not less than a majority of the members of the Board of
Directors of the Company, provided, however, that if the exercise of any such
                          --------  -------
options results in the reduction of the Investor's, or Permitted Transferee's,
ownership to less than a majority, by voting power, of the outstanding capital
stock of the Company, the Company shall so notify the Investor (or Permitted
Transferee), and the Investor or Permitted Transferee shall have the right,
within 30 days after such notice, to purchase from the Company, at a price equal
to the then Fair Market Value (as defined below) thereof, such number of shares
of Common Stock as would increase its ownership to a majority, by voting power,
of the outstanding capital stock of the Company.  "Fair Market Value" shall mean
the average closing price of the Common Stock on the Nasdaq National Market (or
other principal securities exchange or other interdealer quotation system on
which the Common Stock is traded or quoted), during the 10-day period ending on
the day prior to the date of purchase.

          3.2  Termination.  This Article III shall terminate upon the earlier
               -----------
of (i) the sale of all or substantially all of the assets or business of the
Company, by merger, sale of assets or otherwise, and (ii) the first date on
which the Investor (or Permitted Transferee) owns less than a majority, by
voting power, of the outstanding capital stock of the Company for 30 consecutive
days.

          3.3  Permitted Transferee.  For purposes hereof, a "Permitted
               --------------------
Transferee" shall mean any person or entity that acquires directly from the
Investor shares of Common Stock representing at least a majority of the
outstanding shares of Common Stock of the Company and to which the Investor
assigns, in writing, its rights under Section 3.1.  Upon such assignment, the
Permitted Transferee shall be considered the "Investor" for purposes of Section
3.1.

                                       13
<PAGE>

          3.4  Transfers of Rights.  The rights and obligations of the Investor
               -------------------
under Section 2 may be assigned by Investor to any person or entity that
acquires shares of Common Stock having an aggregate value of at least $2,500,000
(as adjusted in stock splits and similar events) from the Investor.  The rights
and obligations of the Investor under Section 3.1 may be assigned only to a
Permitted Transferee, and upon such assignment, the rights and obligations of
the Investor under Section 3.1 shall terminate. In the event of any such
assignment, the assignee must provide written notice of such assignment to the
Company and agree in writing to be bound by the applicable provisions of this
Agreement.





     IV.  General.
          -------

          4.1  Severability.  The invalidity or unenforceability of any
               ------------
provision of this Agreement shall not affect the validity or enforceability of
any other provision of this Agreement.

          4.2  Specific Performance.  In addition to any and all other remedies
               --------------------
that may be available at law in the event of any breach of this Agreement, each
Investor shall be entitled to specific performance of the agreements and
obligations of the Company hereunder and to such other injunctive or other
equitable relief as may be granted by a court of competent jurisdiction.

          4.3  Governing Law.  This Agreement shall be governed by and construed
               -------------
in accordance with the internal laws of the State of Delaware (without reference
to the conflicts of law provisions thereof).

          4.4  Notices.  All notices, requests, consents and other
               -------
communications under this Agreement shall be in writing and shall be deemed
delivered (i) two business days after being sent by registered or certified
mail, return receipt requested, postage prepaid, or (ii) one business day after
being sent via a reputable nationwide overnight courier service guaranteeing
next business day delivery, in each case to the intended recipient as set forth
below:

                                       14
<PAGE>

     If to the Company, at AltaVista Company, 529 Bryant Street, Palo Alto,
California 94301, Attention:  General Counsel, or at such other address or
addresses as may have been furnished in writing by the Company to the Investor;
or

     If to the Investor, at CMGI, Inc., 100 Brickstone Square, Andover,
Massachusetts 01810, or at such other address or addresses as may have been
furnished to the Company in writing by such Investor.

     Any party may give any notice, request, consent or other communication
under this Agreement using any other means (including, without limitation,
personal delivery, messenger service, telecopy, first class mail or electronic
mail), but no such notice, request, consent or other communication shall be
deemed to have been duly given unless and until it is actually received by the
party for whom it is intended. Any party may change the address to which
notices, requests, consents or other communications hereunder are to be
delivered by giving the other parties notice in the manner set forth in this
Section.

          4.5  Complete Agreement.  This Agreement constitutes the entire
               ------------------
agreement and understanding of the parties hereto with respect to the subject
matter hereof and supersedes all prior agreements and understandings relating to
such subject matter.

          4.6  Amendments and Waivers.  Any term of this Agreement may be
               ----------------------
amended or terminated and the observance of any term of this Agreement may be
waived (either generally or in a particular instance and either retroactively or
prospectively) with the written consent of the Company and the Investor.

          4.7  Pronouns.  Whenever the context may require, any pronouns used in
               --------
this Agreement shall include the corresponding masculine, feminine or neuter
forms, and the singular form of nouns and pronouns shall include the plural, and
vice versa.

          4.8  Counterparts; Facsimile Signatures.  This Agreement may be
               ----------------------------------
executed in two counterparts, each of which shall be deemed to be an original,
and both of which together shall constitute one and the same document. This
Agreement may be executed by facsimile signatures.

          4.9  Section Headings.  The section headings are for the convenience
               ----------------
of the parties and in no way alter, modify, amend, limit or restrict the
contractual obligations of the parties.

                                       15
<PAGE>

          4.10 Effective Date.  This Agreement shall become effective upon the
               --------------
closing of the Company's initial public offering of Common Stock pursuant to an
effective registration statement and shall terminate if such offering does not
close prior to July 31, 2000.

                                       16
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the date first written above.

                              ALTAVISTA COMPANY


                              By_______________________________________
                                Name:  Rodney W. Schrock
                                Title: President and  Chief Executive
                                       Officer


                              CMGI, INC.


                              By_______________________________________
                                Name:
                                Title:

                                       17