<PAGE>   1
                                                                    EXHIBIT 4.08




                      SIXTH AMENDMENT TO CREDIT AGREEMENT

Harris Trust and Savings Bank
Chicago, Illinois

Ladies and Gentlemen:

         Reference is hereby made to that certain Credit Agreement (as amended,
supplemented, modified or restated from time to time, the "Credit Agreement"),
dated as of September 23, 1997, by and among Comshare, Incorporated (the
"Company") and Comshare Limited (the "Borrowing Subsidiary") (together the
"Borrowers") and Harris Trust and Savings Bank (the "Bank"). All capitalized
terms used herein without definition shall have the same meanings herein as such
terms have in the Credit Agreement.

         The Borrowers have requested that the Bank extend the Termination Date
and the Bank will do so under the terms and conditions set forth in this
Amendment.

         1. AMENDMENT.

         The definition of "Termination Date" appearing in Section 5 of the
Credit Agreement shall be amended and as so amended shall be restated to read as
follows:

                  "Termination Date" means September 30, 2003, or such earlier
                  date on which the Commitment is terminated in whole pursuant
                  to Section 3.4, 9.2 or 9.3 hereof, or such later date to which
                  the Termination Date is extended pursuant to Section 12.4
                  hereof.

         2. CONDITIONS PRECEDENT.

         The effectiveness of this Amendment is subject to the satisfaction of
all of the following conditions precedent:

                  (a) The Borrowers and the Bank shall have executed and
         delivered this Amendment.

                  (b) The Guarantors and each party signatory to that certain
         Debt Subordination Agreement dated September 23, 1997 shall have each
         executed and delivered to the Bank their consent to this Amendment in
         the forms set forth below.

                  (c) Legal matters incident to the execution and delivery of
         this Amendment shall be satisfactory to the Bank and its counsel.

         3. REPRESENTATIONS.

         In order to induce the Bank to execute and deliver this Amendment, the
Borrowers hereby represent to the Bank that as of the date hereof the
representations and warranties set forth

<PAGE>   2


in Section 6 of the Credit Agreement are and shall be and remain true and
correct (except that the representations contained in Section 6.5 shall be
deemed to refer to the most recent financial statements of the Borrowers
delivered to the Bank) and, except as waived herein, the Borrowers are in
compliance with the terms and conditions of the Credit Agreement and no Default
or Event of Default has occurred and is continuing under the Credit Agreement or
shall result after giving effect to this Amendment.

         4. MISCELLANEOUS.

            4.1. The Borrowers heretofore executed and delivered to the Bank
the Security Agreement, Pledge Agreement and certain other Collateral Documents.
The Borrowers hereby acknowledge and agree that the Liens created and provided
for by the Collateral Documents continue to secure, among other things, the
Obligations arising under the Credit Agreement as amended hereby; and the
Collateral Documents and the rights and remedies of the Bank thereunder, the
obligations of the Borrowers thereunder, and the Liens created and provided for
thereunder remain in full force and effect and shall not be affected, impaired
or discharged hereby. Nothing herein contained shall in any manner affect or
impair the priority of the liens and security interests created and provided for
by the Collateral Documents as to the indebtedness which would be secured
thereby prior to giving effect to this Amendment.

            4.2. Except as specifically amended herein, the Credit Agreement
shall continue in full force and effect in accordance with its original terms.
Reference to this specific Amendment need not be made in the Credit Agreement,
the Note, or any other instrument or document executed in connection therewith,
or in any certificate, letter or communication issued or made pursuant to or
with respect to the Credit Agreement, any reference in any of such items to the
Credit Agreement being sufficient to refer to the Credit Agreement as amended
hereby.

            4.3. The Borrowers agree to pay on demand all costs and expenses of
or incurred by the Bank in connection with the negotiation, preparation,
execution and delivery of this Amendment, including the fees and expenses of
counsel for the Bank.

            4.4. This Amendment may be executed in any number of counterparts,
and by the different parties on different counterpart signature pages, all of
which taken together shall constitute one and the same agreement. Any of the
parties hereto may execute this Amendment by signing any such counterpart and
each of such counterparts shall for all purposes be deemed to be an original.
This Amendment shall be governed by the internal laws of the State of Illinois.

                           [SIGNATURE PAGE TO FOLLOW]




                                       -2-

<PAGE>   3

Dated as of this 17th day of September, 2001.


                                        COMSHARE, INCORPORATED



                                        By /s/ Brian Jarzynski
                                           ------------------------------------
                                          Name Brian Jarzynski
                                              ---------------------------------
                                          Title  VP & CFO
                                               --------------------------------


                                        COMSHARE LIMITED


                                        By /s/ Brian Jarzynski
                                           ------------------------------------
                                          Name Brian Jarzynski
                                              ---------------------------------
                                          Title  Director
                                               --------------------------------


Accepted and agreed to as of the date last above written.


                                        HARRIS TRUST AND SAVINGS BANK


                                        By /s/ Martin McHale
                                           ------------------------------------
                                          Name Martin McHale
                                              ---------------------------------
                                          Title  Vice President
                                               --------------------------------


<PAGE>   4

                     GUARANTORS' ACKNOWLEDGEMENT AND CONSENT

         Each of the undersigned Guarantors heretofore executed and delivered
to the Bank a separate Guaranty Agreement each dated September 23, 1997. Each of
the undersigned hereby consent to the Sixth Amendment to Credit Agreement as set
forth above and confirm that its Guaranty Agreement and all of the undersigned's
obligations thereunder remain in full force and effect. Each of the undersigned
further agree that the consent of the undersigned to any further amendments to
the Credit Agreement shall not be required as a result of this consent having
been obtained, except to the extent, if any, required by the Guaranty Agreement
referred to above.

                                        COMSHARE (U.S.), INC.
                                        COMSHARE LIMITED (CANADA)
                                        COMSHARE HOLDINGS COMPANY



                                        By  /s/ Brian Jarzynski
                                            -----------------------------------
                                          Name: Brian Jarzynski
                                               --------------------------------
                                          Title: VP & CFO
                                                -------------------------------

<PAGE>   5

                SUBORDINATED CREDITORS' ACKNOWLEDGEMENT AND CONSENT

         Each of the undersigned heretofore executed in favor of the Bank a
Debt Subordination Agreement dated September 23, 1997. Each of the undersigned
hereby consent to the Sixth Amendment to Credit Agreement as set forth above and
confirms that the Debt Subordination Agreement and all of the undersigned's
obligations thereunder remain in full force and effect. Each of the undersigned
further agree that the consent of the undersigned to any further amendments to
the Credit Agreement shall not be required as a result of this consent having
been obtained, except to the extent, if any, required by the Debt Subordination
Agreement referred to above.

                                        COMSHARE, INCORPORATED
                                        COMSHARE (U.S.), INC.
                                        COMSHARE LIMITED
                                        COMSHARE HOLDINGS COMPANY
                                        COMSHARE LIMITED (CANADA)



                                        By  /s/ Brian Jarzynski
                                            -----------------------------------
                                          Name: Brian Jarzynski
                                               --------------------------------
                                          Title: VP & CFO
                                                -------------------------------