<PAGE>

                                                                   EXHIBIT 10.18

     THE SECURITIES EVIDENCED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
     THE SECURITIES ACT OF 1933 (THE "ACT") AND MAY NOT BE OFFERED, SOLD OR
     OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED
     UNDER THE ACT OR, IN THE OPINION OF COUNSEL IN FORM AND SUBSTANCE
     SATISFACTORY TO COMPANY, SUCH TRANSACTION IS IN COMPLIANCE THEREWITH.

                        SERIES B PREFERRED STOCK WARRANT

                                       of

                              ATRIEVA CORPORATION

    THIS CERTIFIES THAT ROBERT KINGSBOOK (the "Holder") is entitled to purchase
                                               ------
from ATRIEVA CORPORATION, a Delaware corporation (the "Company"), during the
                                                       -------
Exercise Period, Nine Hundred (900) fully-paid and non-assessable shares of
Company's Series B Preferred Stock (the "Preferred"), at a price of Five Dollars
                                         ---------
($5.00) per share (the "Warrant Price"), such price and number of shares being
                        -------------
subject to adjustment as set forth herein. The "Exercise Period" commences on
                                                ---------------
August 25, 1999 (the "Effective Date") and terminates on the later of (i) the
                      --------------
tenth (l0th) anniversary of the Effective Date, or (ii) the fifth (5th)
anniversary of the IPO. "IPO" means the first Public Offering and "Public
                         ---                                       ------
Offering" means a public offering of Company's Common Stock pursuant to an
--------
effective registration statement under the Act.

     This Warrant is subject to the following terms and conditions:

     1.    Exercise of Warrant. Holder may exercise this Warrant in whole or in
           -------------------
part, at any time during the Exercise Period, by surrendering this Warrant
together with the "Notice of Exercise" and "Investment Representation
                   ------------------       -------------------------
Statement" attached hereto as Exhibits "A" and "B", respectively, duly completed
---------                     --------------------
and executed, at Company's address set forth below its signature hereto (the

"Principal Office") and by paying Company, in the manner provided for in the
-----------------
following paragraph, the Warrant Price for the Preferred purchased.

          In lieu of exercising this Warrant for cash, Holder may elect to
receive shares equal to the value (as determined below) of this Warrant (or the
portion thereof being cancelled) by surrendering this Warrant at the Principal
Office, together with notice of such election (which shall state the number of
shares being exercised hereunder), in which event Company shall issue to Holder
the number of shares of Preferred (or Common Stock if the Preferred has been
converted into Common Stock) equal to the quotient obtained by dividing (x) the
value of the shares of Preferred being exercised (the "Exercised Shares") on the
                                                       ----------------
date this Warrant is exercised (the "Exercise Date"), which shall be determined
                                     -------------
by subtracting (A) the aggregate Warrant Price of the Exercised Shares
immediately prior to the exercise hereof from (B) the aggregate fair market
value of the Exercised Shares on the Exercise Date, by (y) the fair market value
of one share of Preferred (or Common Stock if the Preferred has been converted
into Common Stock) as of the Exercise Date. If the number of shares to be issued
determined in accordance with the foregoing formula is other than a whole
number, Company shall pay to Holder cash equal to the fair market value of the
resulting fractional share on the Exercise Date in lieu of issuing such
fractional share.



                                       1
<PAGE>

          For purposes of converting this Warrant in accordance with the
preceding paragraph, the fair market value per share of the Preferred (or Common
Stock if the Preferred has been converted into Common Stock) shall be determined
as follows:

     (i) If this Warrant is exercised in connection with and contingent upon a
Public Offering, and if Company's registration statement relating to such Public
Offering has been declared effective by the Securities and Exchange Commission,
then the fair market value shall be the initial "Price to Public" specified in
the final prospectus with respect to such offering.

     (ii) If this Warrant is exercised in connection with any Sale or Merger,
then the fair market value shall be equal to the sum of all cash, stock and
other consideration received by Company (as valued in accordance with the
following paragraphs of this section), divided by the number of outstanding
shares (and options to purchase shares) of Company's capital stock (on an as-
converted into Common Stock basis) as of the closing of such transaction.

If this Warrant is not exercised in connection with and contingent upon a Public
Offering, a Sale or Merger, then Company's Board of Directors, acting in good
faith, shall determine fair market value. In determining fair market value, no
discount shall be taken for the shares representing a minority interest in
Company.

     The exercise of this Warrant may be made contingent upon (i) the closing of
a Public Offering, (ii) the closing of any consolidation or merger of Company
with or into any other unaffiliated party or any other reorganization in which
the holders of the Company's voting equity securities immediately prior to such
transaction own less than a majority interest of the successor corporation
following such transaction (a "Merger"), or (iii) the sale of all or
                               ------
substantially all of Company's assets (a "Sale"). Company shall notify Holder if
                                          ----
an event or transaction of the kind described in this section is proposed at
least fifteen (15) days prior to the closing of such event or transaction; such
notice shall also contain such details of the proposed event or transaction as
are reasonable in the circumstances. Notwithstanding the period of
exercisability stated on the face of this Warrant, this Warrant shall become
forever null and void to the extent not exercised before 5:00 p.m. Pacific Time
on the tenth day following the delivery of such notice relating to a Merger or
Sale.

     Certificates for the shares issuable upon exercise of this Warrant and, if
applicable, a new warrant evidencing the balance of the shares remaining subject
to this Warrant shall be issued as of the Exercise Date and shall be delivered
to Holder within thirty (30) days following the Exercise Date (subject to the
transfer restrictions contained herein and upon Holder paying applicable
transfer taxes). All shares of Preferred issued upon the exercise hereof shall
be fully paid and non-assessable and shall be free from all taxes, liens and
charges with respect thereto except for those created by Holder.

     2.   Transfer and Exercise Conditions. This Warrant may be transferred or
          --------------------------------
exercised only if (i) Company receives, at the time of such transfer or
exercise, a representation in writing that this Warrant (or portion hereof
transferred) or the shares of Preferred or other securities being issued upon
such exercise, as applicable, are being acquired for investment not with a view
to any sale or distribution thereof, or a statement of the pertinent facts
covering any proposed distribution thereof, and (ii) other than a transfer
registered under the Act, Company receives a legal opinion, in form and
substance satisfactory to it, reciting the pertinent circumstances surrounding
the proposed transfer and stating that it is exempt from the Act's prospectus
and the registration requirements. The opinion requirement shall not apply to a
transfer to an affiliate of Holder, so long as it complies with applicable
securities laws and affiliate is an accredited investor. Each certificate
evidencing the shares of Preferred issued upon

                                2
<PAGE>

exercise of this Warrant, or Common Stock issued upon conversion of such
Preferred, or upon any transfer of such shares (other than a transfer registered
under the Act or any subsequent transfer of shares so registered) shall, at
Company's option, contain a legend, in form and substance satisfactory to
Company, restricting the transfer of such shares to sales or other dispositions
exempt from the Act's requirements.

     3.   Adjustment of Warrant Price and Shares. The Warrant Price and the
          --------------------------------------
number of shares purchasable hereunder shall be adjusted from time to time as
follows:

          (a) Subdivisions or Combinations. If outstanding shares of the
              ----------------------------
Preferred are subdivided, the Warrant Price in effect immediately prior to such
subdivision shall be proportionately decreased, and if the outstanding shares of
the Preferred are combined, the Warrant Price in effect immediately prior to
such combination shall be proportionately increased, effective at the close of
business on the date of such subdivision or combination, as applicable.

          (b) Stock Dividends. If a dividend is paid with respect to Preferred
              ---------------
in Preferred, then the Warrant Price in effect immediately prior to the record
date for distribution of such dividend shall be adjusted to the price determined
by multiplying the Warrant Price in effect immediately prior to such date by a
fraction (i) the numerator of which is the total number of shares of Preferred
outstanding immediately prior to such dividend and (ii) the denominator of which
is the total number of shares of Preferred outstanding immediately after such
dividend.

          (c) Reclassification. In case of any reclassification, change or
              ----------------
conversion of securities of the class or series issuable upon exercise hereof
(other than as a result of a Merger or Sale or a subdivision or combination
described above), or in case of any Merger or Sale where the successor entity is
obligated to assume or agrees to assume the obligations of this Warrant, Company
or such successor entity, as applicable, shall duly execute and deliver to
Holder a new warrant so that Holder shall have the right to receive, at a total
purchase price not to exceed that payable upon the exercise of the unexercised
portion of this Warrant, and in lieu of the shares of Preferred therefore
issuable upon exercise of this Warrant, the kind and amount of shares of stock,
other securities, money and property receivable upon such reclassification,
change or conversion by a holder of the number of shares of Preferred then
purchasable under this Warrant. Such new warrant shall provide for adjustments
that shall be as nearly equivalent as may be practicable to the adjustments
provided for in this section. The provisions of this subsection shall similarly
apply to successive reclassifications, changes, and conversions.

          (d)  Antidilution Rights. The Preferred's antidilution rights are set
               -------------------
forth in its Certificate of Incorporation attached hereto as Exhibit "C" (the
                                                             -----------
"Certificate"). Such rights shall not be restated, amended or modified in any
------------
manner which adversely affects Holder differently than holders of Preferred
without Holder's prior written consent. Company shall promptly notify Holder of
any restatement, amendment or modification to the Certificate.

          (e)  Notices. Within thirty (30) days after each adjustment of the
               -------
Warrant Price and the number of shares of Preferred purchasable hereunder,
Company shall give written notice to Holder of the adjusted Warrant Price and
the increased or decreased number of shares purchasable hereunder, setting forth
in reasonable detail the method of calculation of each.

     4.  Registration.  Holder shall become a party to the Company's Third
         ------------
Amended and Restated Investors' Rights Agreement (the "Rights Agreement") and
the Company shall cause the


                                       3
<PAGE>

Amendment to Rights Agreement attached hereto as Exhibit "E" to become effective
                                                ------------
and binding on the parties thereto. Holder shall also become a party to the
Company's Series A-1 and Series B Preferred Stock Voting Agreement, attached
hereto as Exhibit "F."
          ------------


     5.  Holder Representations. Concurrently herewith, Holder shall have
         ----------------------
executed the Investment Representation Statement.

     6.  Company Representations. Company represents and warrants to Holder, as
         -----------------------
of the Effective Date:

          (a)  This Warrant has been duly authorized and executed by Company and
is a valid and binding obligation of Company, enforceable in accordance with its
terms except as to the effect of (i) applicable bankruptcy and similar laws
affecting the rights of creditors, and (ii) rules of law governing specific
performance, injunctive relief and other equitable remedies.

          (b)  The Preferred, when issued in accordance with the terms hereof,
will be validly issued, fully paid and non-assessable, and free from all taxes,
liens and charges with respect to the issue of such shares.

          (c)  Company's execution and delivery of this Warrant does not, and
the issuance of the Preferred upon exercise of this Warrant in accordance with
the terms hereof does not, (i) conflict with Company's Certificate or Bylaws,
(ii) contravene any law, governmental rule or regulation, judgment or order
applicable to Company, or (iii) conflict with or constitute a default under any
contract to which Company is a party or by which it is bound or require the
consent or approval of, the giving of notice to, the registration or filing with
or the taking of any action in respect of or by, any federal, state or local
government authority or agency or other person, other than state or federal
securities law filings.

          (d)  During the period within which this Warrant may be exercised,
Company will at all times have authorized and reserved for the purpose of the
issue upon exercise of this Warrant a sufficient number of shares of Preferred
to provide for the exercise of this Warrant and a sufficient number of shares of
Common Stock to provide for the conversion of the Preferred into Common Stock.

     7.   Miscellaneous.
          -------------

          (a)  The terms of this Warrant shall be binding upon and shall inure
to the benefit of the successors or assigns of Company and of Holder and of the
Preferred issued or issuable upon the exercise hereof, and Company's obligations
relating to the Preferred issuable upon exercise of this Warrant shall survive
such exercise.

          (b)  Company stipulates that Holder's remedies at law if Company
defaults or threatens to default in performing in accordance with the terms
hereof are not and will not be adequate to the fullest extent permitted by law,
and that such terms may be specifically enforced by a decree for the specific
performance of any agreement contained herein or by an injunction against a
violation of any of the terms hereof or otherwise. If either party seeks to
enforce its rights hereunder by legal proceedings or otherwise, then the non-
prevailing party shall pay the prevailing party's reasonable costs and expenses,
including all reasonable attorneys' fees.

          (c)  Any communication pursuant hereto will be sufficiently given if
sent by first class mail, postage prepaid, addressed to (a) Holder at its last
known address appearing on Company's books or (b) Company at the Principal
Office. A party may designate a different address (and Company



                                       4
<PAGE>

may change the Principal Office) by notice to the other pursuant to this
subsection. A notice shall be deemed effective upon the earlier of (i) receipt
or (ii) the third day after mailing in accordance with the terms of this
subsection.

          (d)  Company shall not, by amendment of the Certificate or otherwise,
avoid or seek to avoid the observance or performance of any of the terms hereof,
but shall at all times in good faith assist in the carrying out of all such
terms and in the taking of all such action needed or appropriate to protect
Holder's rights against impairment.

          (e)  Upon receipt of evidence reasonably satisfactory to Company of
the loss, theft, destruction or mutilation of this Warrant and, in the case of
any such loss, theft or destruction, upon delivery of an indemnity agreement
reasonably satisfactory in form and amount to Company, or in the case of any
such mutilation, upon surrender and cancellation of such Warrant, Company, at
its expense, shall execute and deliver, in lieu thereof, a new Warrant of like
date and tenor.

          (f)  This Warrant shall be governed by the laws of the State of
Delaware, without giving effect to conflicts of law principles.

          (g)  So long as this Warrant has not terminated, Holder shall be
entitled to receive such financial information as any holders of the Preferred.

     IN WITNESS WHEREOF, Company has caused this Warrant to be signed by its
duly authorized officer.

ATRIEVA CORPORATION                ACCEPTED AND AGREED:
                                   ROBERT KINGSBOOK

By: /s/ KENT W. JARVI               By: /s/ ROBERT A. KINGSBOOK
    -----------------                   -----------------------

Print Name: Kent W. Jarvi          Print Name:  Robert A. Kingsbook
            -------------                       -------------------

Title:  CFO                        Title:
        ---                               ----------------------

Address: 380 Brannan St.
         ---------------

         San Francisco, CA 94107
         -----------------------


                                       5
<PAGE>

                                   EXHIBIT A
                                   ---------

                               NOTICE OF EXERCISE

Ladies and Gentlemen:

The undersigned Holder (the "Holder") elects to exercise the Series B Preferred
                             ------
Stock Warrant (the "Warrant") by and between Holder and ATRIEVA CORPORATION (the
                    -------
"Company"), dated August 25, 1999, by surrendering the Warrant at the Principal
 -------
Office, in exchange for __________ shares of Series B Preferred Stock of Company
(or, ________  shares of Company's Common Stock if such Series B Preferred Stock
has been converted into Common Stock).

Holder confirms the investment representations and warranties made in Investment
Representation Statement of the Warrant, a copy of which is available from
Company, and accepts such shares subject to the restrictions of the Warrant.

Dated: __________________________________________

HOLDER: _______________________________________

________________________________________________
(Signature)

________________________________________________
(Typed or Printed Name)

________________________________________________
(Title)

Address:

________________________________________________
________________________________________________
________________________________________________
<PAGE>

                                   EXHIBIT B
                                   ---------

                      INVESTMENT REPRESENTATION STATEMENT

                              Warrants to Purchase
                     _________ Shares of Series B Preferred
                          Stock of ATRIEVA CORPORATION

     In connection with the purchase of the above-listed securities the
undersigned hereby represents to ATRIEVA CORPORATION] ("Company"):
                                                        -------

     1 .  Receipt of Information.  It has received all the information it
          ----------------------
considers necessary or appropriate for deciding whether to purchase Company's
Series B Preferred Stock issuable upon exercise of the Warrant, dated __________
(the "Warrant"), issued by Company to it, and it has examined any information
      -------
furnished to it by Company in connection therewith.

     2.   Investment Representation.
          -------------------------

     (a) The Warrant and the shares of stock to be received by it upon
exercising the Warrant (the "Securities") will be acquired for investment for
                             ----------
its own account, not as a nominee or agent, and not with a view to the sale or
distribution of any part thereof and it has no present intention of selling,
granting participation in or otherwise distributing the same, but subject,
nevertheless, to any requirement of law that the disposition of its property
shall at all times be within its control. By executing this Statement, it
further represents that it does not have any contract, undertaking, agreement or
arrangement with any person to sell, transfer, or grant participations with
respect to any Securities.

     (b) It understands that the Securities may not be registered under the
Securities Act of 1933, as amended (the "Act"), and applicable state securities
                                         ---
laws, on the ground that the issuance of such Securities is exempt pursuant to
Section 4(2) of the Act and state law exemptions relating to offers and sales
not by means of a public offering, and that Company's reliance on such
exemptions is predicated on the undersigned's representations set forth herein.
It is an "Accredited Investor", as defined in Rule 501 of the Securities and
          -------------------
Exchange Commission.

     (c) It will not make a disposition of any Securities until it has (i)
notified Company of the proposed disposition and has furnished Company with a
statement of the circumstances surrounding the proposed disposition, and (ii)
has furnished Company with an opinion of counsel satisfactory to Company and
Company's counsel to the effect that (a) appropriate action for complying with
the Act and any applicable state securities laws has been taken or an exemption
from the registration requirements of the Act and such laws is available, and
(b) the proposed transfer will not violate any of said laws.

     (d) It is able to fend for itself in the transactions contemplated by this
Statement, has such knowledge and experience in financial and business matters
as to be capable of evaluating the merits and risks of its investments, and has
the ability to bear the economic risks (including the risk of a total loss) of
its investment. It has had the opportunity to ask questions of Company
concerning Company's business and assets and to obtain any additional
information which it considered necessary to verify the accuracy of or to
amplify Company's disclosures, and has had all questions which have been asked
by it satisfactorily answered by Company.


                                       1
<PAGE>

     (e)  It acknowledges that the Securities must be held indefinitely unless
subsequently registered under the Act or an exemption from such registration is
available. It is aware of the provisions of Rule 144 promulgated under the Act
which permit limited resale of shares purchased in a private placement subject
to the satisfaction of certain conditions, including, among other things, the
existence of a public market for the shares, the availability of certain current
public information about Company, the resale occurring not less than one year
after a party has purchased and paid for the security to be sold, the sale being
through a "broker's transaction" or in transactions directly with a "market
maker" (as provided by Rule 144(f)) and the number of shares being sold during
any three-month period not exceeding specified limitations. It is aware that the
conditions for resale set forth in Rule 144 have not been satisfied.

     (f)  It represents that at no time was it presented with or solicited by
any publicly issued or circulated newspaper, mail, radio, television or other
form of general advertising or solicitation in connection with the offer, sale
and purchase of the Securities.

     (g) If it is the original holder of the Warrant, it has a preexisting
business or personal relationship with Company or any of its officers, directors
or controlling persons, or by reason of its business or financial experience or
the business or financial experience of its professional advisors who are
unaffiliated with and who are not compensated by Company or any affiliate or
selling agent of Company, directly or indirectly, has, and could be reasonably
assumed to have, the capacity to protect its own interests in connection with
the purchase of the Securities.

Dated:___________________________________________

________________________________________________
(Signature)

________________________________________________
(Typed or Printed Name)

________________________________________________
(Title)


                                       2
<PAGE>

                                   EXHIBIT C
                                   ---------

                          CERTIFICATE OF INCORPORATION
<PAGE>

                                   EXHIBIT D
                                   ---------

                                RIGHTS AGREEMENT
<PAGE>

                                   EXHIBIT E
                                   ---------

                         AMENDMENT TO RIGHTS AGREEMENT