<PAGE>
                                                                   Exhibit 10.55

                                PROMISSORY NOTE

$ 192,899.70                                        DECEMBER 21, 2000
                                                    WESTBORO, MASSACHUSETTS

     For value received, William P. Ferry (the "Borrower") promises to pay to
the order of ePresence, Inc. (the "Company") at the offices of the Company or
such other place as the Company hereafter shall designate One Hundred Ninety Two
Thousand Eight Hundred Ninety Nine Dollars and Seventy Cents ($192,899.70) to
satisfy his federal and state tax obligations with respect to the issuance to
him of One Hundred Twenty Thousand Shares of restricted common stock of the
Company at a par value of $0.01 per share, pursuant to a Corporate Officer
Restricted Stock Agreement dated December 21, 2000.  This promissory note is
being issued pursuant to Amendment No. 4 to Employment Agreement between the
Borrower and the Company, dated November 16, 2000.  Simple interest shall accrue
on the unpaid balance from time to time outstanding, which interest rate shall
be adjusted each month to the then applicable Federal rate, beginning on the
date hereof.  The principal and all accrued interest of this Note shall be
repaid in full 90 days after the Borrower's employment with the Company is
terminated, unless the Borrower is terminated pursuant to a Change in Control
(as defined in the Employment Agreement between the Borrower and the Company,
dated February 4, 1997), in which case the loan shall be due and payable in full
180 days after the Borrower's termination date.  The Borrower shall repay this
Note prior to the aforementioned dates as provided below.

     At the option of the Company, this Note shall become immediately due and
payable without notice or demand upon the institution by the Borrower of any
proceedings under the United States Bankruptcy Code or the pendency of any
bankruptcy or insolvency proceeding against the Borrower for a period of 60 days
from the institution thereof or the making by the Borrower of an assignment for
the benefit of creditors (the "Event of Default").

     Upon the occurrence of the Event of Default, the Company shall have then,
or at any time thereafter, all of the rights and remedies afforded by the
Uniform Commercial Code as from time to time in effect in the Commonwealth of
Massachusetts or afforded by other applicable law.

     In no event shall any interest charged, collected or reserved under this
Note exceed the maximum rate then permitted by applicable law and if any such
payment is paid by the holder as a payment of principal.

     All payments by the Borrower under this Note shall be made without set-off
or counterclaim and be free and clear and without deduction or withholding for
any taxes or fees of any nature whatever, unless the obligation to make such
deduction or withholding is imposed by law.  The Borrower shall pay and save the
holder harmless from all liabilities with respect to or resulting from any delay
or omission to make any such deduction or withholding required by law.

                                       1
<PAGE>

     Whenever any amount is paid under this Note, all or part of the amount paid
may be applied to principal, interest, premium or interest in such order and
manner as shall be determined by the holder in its discretion.

     No reference in this Note to any other document shall impair the obligation
of the Borrower, which is absolute and unconditional, to pay all amounts under
this Note strictly in accordance with the terms of this Note.

     No delay or omission on the part of the Company in exercising any right
hereunder shall operate as a waiver of such right or of any other right of such
Company, nor shall any delay, omission or waiver on any one occasion be deemed a
bar to or waiver of the same or any other right on any future occasion.

     The Borrower waives presentment, demand, notice, protest and all other
demands and notices in connection with the delivery, acceptance, performance,
default or enforcement hereof and also waives any delay on the part of the
Company.  No discharge or release of any other party primarily or secondarily
liable hereon shall discharge or otherwise affect the liability of the Borrower
to the Company.

     None of the terms or provisions of this Note may be excluded, modified, or
amended except by a written instrument duly executed on behalf of the Borrower
expressly referring hereto and setting forth the provision so excluded, modified
or amended.

     The Borrower will pay on demand all costs of collection, including
reasonable attorney's fees, incurred by the Company in enforcing the obligations
created by this Note.

     This Note shall be binding upon the Borrower and each endorser and
guarantor hereof and upon their heirs, successors and representatives, and shall
inure to the benefit of the Company and its successors, endorsees and assigns.

     This Note shall be governed by, and construed and enforced in accordance
with, the substantive laws of The Commonwealth of Massachusetts, without regard
to its principled of conflicts of laws.  This Note shall be deemed to be under
seal.


WITNESS:                                    BORROWER:

Signed in my Presence:


/s/ Richard M. Spaulding                    /s/ William P. Ferry
---------------------------------------     -----------------------------------
Richard M. Spaulding                        William P. Ferry

                                       2