<PAGE>

                                                                    EXHIBIT 10.4


THIS NOTE AND ANY SECURITIES ISSUABLE UPON THE CONVERSION OF THIS NOTE HAVE NOT
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE
TRANSFERRED OR OTHERWISE DISPOSED OF UNLESS THEY HAVE BEEN REGISTERED UNDER SUCH
ACT OR PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER SUCH ACT.

THE PAYMENT OF THIS NOTE AND THE RIGHTS OF THE HOLDER OF THIS NOTE ARE
SUBORDINATED TO THE PAYMENT OF SUPERIOR INDEBTEDNESS AND THE RIGHTS OF THE
HOLDERS OF SUPERIOR INDEBTEDNESS UPON THE TERMS OF SUBORDINATION SET FORTH IN
THAT CERTAIN SUBORDINATION AGREEMENT DATED DECEMBER 12, 2000 BY AMONG THE ISSUER
OF THIS NOTE AND CMGI, INC., AS AMENDED, MODIFIED AND SUPPLEMENTED FROM TIME TO
TIME.

                                NAVISITE, INC.

                     7.5% Convertible, Subordinated Note
                             Due December 12, 2003

U.S. $50,000,000                                               December 12, 2000


          FOR VALUE RECEIVED, the undersigned, NaviSite, Inc., a Delaware
corporation (the "Borrower"), hereby promises to pay to CMGI, Inc., a Delaware
corporation (the "Lender"), the principal sum of Fifty Million Dollars
($50,000,000) (the "Principal Sum"), or such lesser amount as may then be
outstanding, together with accrued but unpaid interest thereon, on December 12,
2003 (the "Maturity Date"). Interest on the outstanding Principal Sum shall be
at a rate of 7.5% per annum ("Interest"), calculated on the basis of a 360 day
year consisting of twelve 30 day months and payable quarterly in arrears on
January 31, April 30, July 31 and October 31 of each year until this Note is
paid in full (each an "Interest Payment Date"). If all or a portion of the
Principal Sum or Interest shall not be paid when due (whether at its stated
maturity, by acceleration or otherwise), the Borrower hereby promises to pay, on
demand, interest on such overdue amount from and including the due date to, but
excluding, the date such amount is paid in full at 10.5% per annum (and until
the date such overdue amount is paid in full, "Interest" on such overdue amount
shall mean interest at such rate).

          This Note is being delivered pursuant to that certain Note and Warrant
Purchase Agreement, dated as of December 12, 2000, between the Borrower and the
Lender (the "Note and Warrant Purchase Agreement"). All capitalized terms used
<PAGE>

but not otherwise defined herein shall have the meaning ascribed to such terms
in the Note and Warrant Purchase Agreement.

1.   Subordination.

     The Lender's right to payment pursuant to the terms of this Note shall be
subordinated to the extent and on the terms and conditions set forth in the
Subordination Agreement, dated as of December 12, 2000, by and among the Lender
and the Borrower (the "Subordination Agreement"), and the terms and provisions
of the Subordination Agreement are incorporated herein by reference.

2.   Payment.

          Payment of the Principal Sum on the Maturity Date and Interest on each
Interest Payment Date shall be made by certified or bank cashier's check payable
to the Lender at the Lender's principal address set forth in Section 6 hereof
(or at such other place as the Lender hereof shall notify the Borrower in
writing) or, if the Lender so specifies, by written notice to the Borrower given
not less than two Business Days prior to the Maturity Date or the Interest
Payment Date, as the case may be, by bank wire transfer, in immediately
available funds, to the account so specified, in lawful money of the United
States of America, provided that, at the election of the Borrower, the amount of
                   -------- ----
Interest due on any Interest Payment Date may be paid in shares of common stock,
par value $0.01 per share ("Common Stock"), of the Borrower, with the number of
shares of Common Stock to be issued in payment of the Interest due on such
Interest Payment Date to equal the amount of such Interest divided by the
average of the closing prices per share of the Common Stock on the Nasdaq
National Market as reported in The Wall Street Journal on the five consecutive
                               -----------------------
trading days ending on the trading day immediately preceding such Interest
Payment Date. The Borrower may prepay this Note at any time, without premium or
penalty, in whole or in part, with accrued interest to the date of such payment
on the amount prepaid. If the Maturity Date or any Interest Payment Date occurs
on a date that is not a Business Day then the Principal Sum or Interest then due
shall be paid on the next succeeding Business Day. "Business Day" shall mean any
day other than Saturday, Sunday or any day upon which banks in the city of
Boston, Massachusetts are authorized or required to be closed.

3.   Conversion.

     (a)  Conversion Rights. Subject to and in accordance with the provisions of
          -----------------
this Section 3, at any time on or prior to the Maturity Date, the Lender may
elect, in its sole discretion, to effect the conversion (the "Conversion") of
all or any portion of the Principal Sum and Interest due on this Note into
Common Stock. The number of shares of Common Stock into which the Principal Sum
and accrued but unpaid

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<PAGE>

Interest (or portion thereof) shall be converted pursuant to this Section 3(a)
shall be determined by dividing the amount of Principal Sum and Interest the
Lender has elected to convert by the Conversion Price. The Conversion Price is
subject to adjustment as provided in Section 4 hereof.

     (b)  Mandatory Conversion. Subject to the provisions of Section 3(c)
          --------------------
hereof, if there is a Mandatory Conversion Event (as defined below) at any time
on or prior to the Maturity Date, the Borrower shall have the right to require
the conversion of all, but not less than all, of the outstanding Principal Sum
and Interest into Common Stock at the Conversion Price (the "Mandatory
Conversion") in accordance with Sections 3(a) and 3(c) hereof.

     (c)  Manner of Effecting the Conversion. If the Lender elects to effect the
          ----------------------------------
Conversion pursuant to Section 3(a) hereof, the Lender shall deliver a duly
executed written notice to the Borrower of such election (the "Conversion
Notice"), and in such event the Conversion shall be deemed to have been effected
at the close of business on the date such Conversion Notice is given. If the
Borrower elects to effect the Mandatory Conversion pursuant to Section 3(b)
hereof, the Borrower shall deliver a duly executed written notice to the Lender
(the "Mandatory Conversion Notice") within ten Business Days of any period of 20
consecutive trading days during which the closing price per share of the Common
Stock as reported on the Nasdaq National Market exceeds 175% of the Conversion
Price on at least 15 trading days (a "Mandatory Conversion Event"), and in such
event the Mandatory Conversion shall be deemed to have been effected at the
close of business on the date such Mandatory Conversion Notice is given. Upon
any Conversion or Mandatory Conversion of this Note in accordance with the terms
hereof, the rights of the Lender with respect to the Principal Sum and all
Interest pursuant to this Note shall cease and the Lender shall be deemed to
have become the holder of record of the shares of Common Stock into which this
Note shall have been converted, provided that, if the Lender elects to convert
                                -------- ----
only a portion of the Principal Sum and Interest pursuant to Section 3(a)
hereof, then the Borrower will deliver a new note to the Lender, on the same
terms and conditions as this Note, with respect to the portion of the Principal
Sum and Interest that is not converted (the "New Note"). Concurrently with the
delivery of a Conversion Notice or receipt of a Mandatory Conversion Notice, as
the case may be, the Lender shall surrender this Note to the Borrower. Promptly
upon its receipt of a Conversion Notice or delivery of a Mandatory Conversion
Notice, as the case may be, the Borrower shall (i) deliver to or upon the
written order of the Lender, a certificate or certificates for the number of
shares of Common Stock issuable upon such Conversion or Mandatory Conversion,
(ii) make a cash payment in respect of any fraction of a share as provided in
Section 3(d) hereof and (iii) if applicable, deliver a New Note as set forth in
this Section 3(c).

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<PAGE>

     (d)  Fractional Shares. No fractional shares shall be issued upon any
          -----------------
Conversion or Mandatory Conversion. Instead of any fractional share which would
otherwise be issuable upon a Conversion or Mandatory Conversion, the Borrower
shall pay a cash amount in respect of such fractional share in an amount based
upon the Conversion Price.

4.   Antidilution Provisions.

     (a)  Reorganization, Reclassification or Recapitalization of the Borrower.
          --------------------------------------------------------------------
In case of (i) a capital reorganization, reclassification or recapitalization of
the Borrower's capital stock (other than in the cases referred to in Section
4(c) hereof), (ii) the Borrower's consolidation or merger with or into another
corporation in which the Borrower is not the surviving entity, or a merger in
which the Borrower is the surviving entity but the shares of the Borrower's
capital stock outstanding immediately prior to the merger are converted, by
virtue of the merger, into other property, whether in the form of securities,
cash or otherwise, or (iii) the sale or transfer of all or substantially all of
the Borrower's assets, then, as part of such reorganization, reclassification,
recapitalization, merger, consolidation, sale or transfer, lawful provision
shall be made so that there shall thereafter be deliverable upon the Conversion
or Mandatory Conversion of the Principal Sum and Interest or any portion thereof
(in lieu of or in addition to the number of shares of Common Stock theretofore
deliverable, as appropriate) and without payment of any additional
consideration, the number of shares of stock or other securities or property to
which the holder of the number of shares of Common Stock which would otherwise
have been deliverable upon the Conversion or Mandatory Conversion of the
Principal Sum and Interest or any portion thereof at the time of such
reorganization, reclassification, recapitalization, consolidation, merger, sale
or transfer would have been entitled to receive in such reorganization,
reclassification, recapitalization, consolidation, merger, sale or transfer.
This Section 4(a) shall apply to successive reorganizations, reclassifications,
recapitalizations, consolidations, mergers, sales and transfers and to the stock
or securities of any other corporation that are at the time receivable upon the
Conversion or Mandatory Conversion of the Principal Sum and Interest or any
portion thereof. If the per share consideration payable to the Lender for shares
of Common Stock in connection with any transaction described in this Section
4(a) is in a form other than cash or marketable securities, then the value of
such consideration shall be determined in good faith by the Borrower's board of
directors.

     (b)  Splits and Combinations. If the Borrower at any time or from time to
          -----------------------
time after the date of this Note subdivides any of its outstanding shares of
Common Stock into a greater number of shares, the Conversion Price in effect
immediately prior to such subdivision shall be proportionately reduced, and,
conversely, if the outstanding shares of Common Stock are combined into a
smaller number of shares,

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<PAGE>

the Conversion Price in effect immediately prior to such combination shall be
proportionately increased.

     (c)  Reclassifications. If the Borrower reclassifies or otherwise changes
          -----------------
any of the securities into which this Note may be convertible into the same or a
different number of securities of any other class or classes, this Note shall
thereafter be convertible into such number and kind of securities as would have
been issuable as the result of such change with respect to the securities that
this Note was convertible into immediately prior to such reclassification or
other change and the Conversion Price therefor shall be appropriately adjusted.

     (d)  Adjustment Certificates. Upon any adjustment of the Conversion Price
          -----------------------
or the number of shares of Common Stock issuable upon the Conversion or
Mandatory Conversion of this Note, a certificate, signed by (i) the Borrower's
President and Chief Financial Officer or (ii) any independent firm of certified
public accountants of recognized national standing the Borrower selects at its
own expense, setting forth in reasonable detail the events requiring the
adjustment and the method by which such adjustment was calculated, shall be
mailed to the Lender at the address set forth in Section 6 hereof and shall
specify the adjusted Conversion Price and the number of shares of Common Stock
issuable upon the Conversion or Mandatory Conversion of the Note after giving
effect to the adjustment.

     (e)  No Impairment. The Borrower shall not, by amendment of its Certificate
          -------------
of Incorporation or through any reorganization, recapitalization, transfer of
assets, consolidation, merger, dissolution, issue or sale of securities or any
other voluntary action, avoid or seek to avoid the observance or performance of
any of the terms to be observed or performed hereunder by the Borrower, but
shall at all times in good faith assist in the carrying out of all provisions of
this Section 4 and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Lender against impairment. The
Borrower shall not be deemed to have avoided or to be seeking to avoid the
observance or performance of any of the terms to be observed or performed
hereunder by issuing securities after the Closing Date for a consideration per
share less than the Conversion Price then in effect.

     (f)  Application. Except as otherwise provided herein, all sections of this
          -----------
Section 4 are intended to operate independently of one another. If an event
occurs that requires the application of more than one section, all applicable
sections shall be given independent effect.

5.   Default and Remedies.

     (a)  If any of the following events or conditions (each an "Event of
Default") shall occur and be continuing:

                                       5
<PAGE>

          (i)   the Borrower shall fail to pay the Principal Sum when and as the
                same shall become due and payable, whether at its stated
                maturity, by acceleration or otherwise;

          (ii)  the Borrower shall fail to pay any Interest within 30 days of
                when and as the same shall become due and payable, whether at
                its stated maturity, by acceleration or otherwise;

          (iii) upon or after the occurrence of a default with respect to any
                other outstanding indebtedness of the Borrower for borrowed
                money the holder thereof declares all obligations with respect
                to such indebtedness to be due and payable prior to the stated
                maturity or regularly scheduled dates of payment with respect to
                such indebtedness;

          (iv)  if the Stockholders' Meeting is required, the Borrower fails to
                hold the Stockholders' Meeting in accordance with Section 5 of
                the Note and Warrant Purchase Agreement;

          (v)   the Borrower fails to grant and deliver the Warrants in
                accordance with the terms of the Note and Warrant Purchase
                Agreement;

          (vi)  an involuntary proceeding shall be commenced or an involuntary
                petition shall be filed in a court of competent jurisdiction
                seeking (A) relief in respect of the Borrower, or of a
                substantial part of the property or assets of the Borrower,
                under Title 11 of the United States Code, as now constituted or
                hereafter amended, or any successor to or replacement of such
                statute, or any other Federal or state bankruptcy, insolvency,
                receivership or similar law, (B) the appointment of a receiver,
                trustee, custodian, sequestrator, conservator or similar
                official for the Borrower or for a substantial part of the
                properties or assets of the Borrower or (C) the winding-up,
                liquidation or dissolution of the Borrower; and such proceeding
                or petition shall continue undismissed for 90 days or an order
                or decree approving or ordering any of the foregoing shall be
                entered; or

          (vii) the Borrower (A) voluntarily commences any proceeding or files
                any petition seeking relief under Title 11 of the United States
                Code, as now constituted or hereafter amended, or any successor
                to or replacement of such statute, or any other

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<PAGE>

                Federal or state bankruptcy, insolvency, receivership or similar
                law, (B) consents to, or fails to contest in a timely and
                appropriate manner, the commencement against of any proceeding
                or the filing of any petition described in clause (v) above, (C)
                applies for or consents to the appointment of a receiver,
                trustee, custodian, sequestrator, conservator or similar
                official for the Borrower or for a substantial part of the
                properties or assets of the Borrower, (D) files an answer
                admitting the material allegations of a petition filed against
                it in any such proceeding, (E) makes a general assignment for
                the benefit of creditors, (F) becomes unable, admits in writing
                its inability or fails generally to pay its debts as they become
                due or (G) takes any action for the purpose of effecting any of
                the foregoing;

then, (x) in the case of an Event of Default specified in clause (a)(i), (ii),
(iii), (iv) or (v) above, the Lender may, at any time during the continuance of
such Event of Default, by written notice to the Borrower, declare the entire
outstanding Principal Sum, together with all accrued and unpaid Interest, to be
due and payable and (y) in the case of an Event of Default specified in clauses
(a)(vi) or (vii) above, the entire outstanding Principal Sum, together with all
accrued and unpaid Interest, shall automatically forthwith become due and
payable without presentment, protest or notice of any kind, all of which are
hereby expressly waived by the Borrower.

     (b)  Subject to the other terms of this Note, if an Event of Default occurs
and is continuing, the Lender may pursue any available remedy to collect the
payment of the Principal Sum or Interest or to enforce the performance of any
provision of this Note. If an Event of Default occurs and is continuing, the
Lender may proceed to protect and enforce its rights by any action at law, suit
in equity or other appropriate proceeding. In the case of a default in the
payment of the Principal Sum or Interest, the Borrower will pay to the Lender
such further amount as shall be sufficient to cover the costs and expenses of
collection, including, without limitation, reasonable attorneys' fees, expenses
and disbursements.

     (c)  The Borrower shall promptly furnish the Lender notice of any Event of
Default or event which, with notice, lapse of time or both, would become an
Event of Default under this Section 5.

6.   Notices

          All notices, instructions and other communications given hereunder or
in connection herewith shall be in writing. Any such notice, instruction or

                                       7
<PAGE>

communication shall be sent either (i) by certified mail, return receipt
requested, postage prepaid, (ii) via a reputable nationwide overnight courier
service or (iii) by facsimile (with "answer-back" confirmation), in each case to
the address set forth below. Any such notice, instruction or communication shall
be deemed to have been delivered upon receipt of confirmation of delivery after
it is sent by certified mail, return receipt requested, postage prepaid, one
Business Day after it is sent via a reputable nationwide overnight courier
service, or upon receipt of confirmation of delivery of a facsimile.

If to the Borrower to:    NaviSite, Inc.
                          400 Minuteman Road
                          Andover, MA 01810
                          Attention: General Counsel
                          Facsimile: (978) 682-8100


With a copy to:           Hale and Dorr LLP
                          60 State Street
                          Boston, MA 02109
                          Attention: Mark G. Borden, Esq.
                          Facsimile: (617) 526-5000

If to the Lender to:      CMGI, Inc.
                          100 Brickstone Square
                          Andover, MA 01810
                          Attention: William Williams, II, Esq.
                          Facsimile: (978) 684-3601

With a copy to:           Skadden, Arps, Slate Meagher & Flom LLP
                          One Beacon Street
                          Boston, MA 02108
                          Attention: David T. Brewster, Esq.
                          Facsimile: (617) 573-4822

Either party may give any notice, instruction or communication in connection
with this Note using any other means (including personal delivery or ordinary
mail), but no such notice, instruction or communication shall be deemed to have
been delivered unless and until it is received by the party to whom it was sent.
Either party may change the address to which notices, instructions or
communications are to be delivered by giving the other party to this Note notice
thereof in the manner set forth in this Section 6.

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<PAGE>

7.   Miscellaneous.

          This Note shall be construed and enforced in accordance with the laws
of the Commonwealth of Massachusetts, without regard to its conflicts of laws
rules. The Borrower waives presentment, demand, notice, protest and all other
demands and notices in connection with the delivery, acceptance, performance,
default and enforcement of this Note. The Borrower hereby irrevocably and
unconditionally submits, for itself and its property, to the non-exclusive
jurisdiction of the courts of the Commonwealth of Massachusetts and of the
United States District Court for the District of Massachusetts, and any
appellate court of such courts, in any action or proceeding arising out of or
relating to this Note, or for recognition or enforcement of any judgment, and
the Borrower hereby irrevocably and unconditionally agrees that all claims in
respect of any such action or proceeding may be heard and determined in such
Massachusetts court (or, to the extent permitted by law, in such federal court).
The Borrower agrees that a final, unappealable judgment in any such action or
proceeding shall be conclusive and may be enforced in other jurisdictions by
suit on the judgment or in any other manner provided by law. Nothing in this
Note shall affect any right that the Lender may otherwise have to bring any
action or proceeding relating to this Note against the Borrower or its
properties in the courts of any jurisdiction.

THE LENDER AND THE BORROWER AGREE THAT NEITHER OF THEM NOR ANY ASSIGNEE OR
SUCCESSOR SHALL (A) SEEK A JURY TRIAL IN ANY LAWSUIT, PROCEEDING, COUNTERCLAIM
OR ANY OTHER ACTION BASED UPON, OR ARISING OUT OF, THIS NOTE, ANY RELATED
INSTRUMENTS, ANY COLLATERAL OR THE DEALINGS OR THE RELATIONSHIP BETWEEN OR AMONG
ANY OF THEM, OR (B) SEEK TO CONSOLIDATE ANY SUCH ACTION WITH ANY OTHER ACTION IN
WHICH A JURY TRIAL CANNOT BE OR HAS NOT BEEN WAIVED. THE PROVISIONS OF THIS
PARAGRAPH HAVE BEEN FULLY DISCUSSED BY THE LENDER AND THE BORROWER, AND THESE
PROVISIONS SHALL BE SUBJECT TO NO EXCEPTIONS. NEITHER THE LENDER NOR THE
BORROWER HAS AGREED WITH OR REPRESENTED TO THE OTHER THAT THE PROVISIONS OF THIS
PARAGRAPH WILL NOT BE FULLY ENFORCED IN ALL INSTANCES.

          If any provision of this Note shall be held invalid or unenforceable
by any court of competent jurisdiction, that holding shall not invalidate or
render unenforceable any other provision hereof.

          This Note and all rights hereunder are freely transferable by the
holder hereof, subject to compliance with applicable state and federal
securities laws.

                                       9
<PAGE>

          This Note may not be changed, amended or modified except by agreement
in writing signed by the Borrower and the Lender.



           [The remainder of this page is intentionally left blank.]

                                       10
<PAGE>

          IN WITNESS WHEREOF, the Borrower has caused this Note to be signed on
its behalf, in its corporate name, by its duly authorized officer as an
instrument under seal, as of the day and year first above written.

                                   NAVISITE, INC.


                                   By /s/ Joel Rosen
                                     ------------------------------
                                     Name:  Joel Rosen
                                     Title: Chief Executive Officer

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