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                                                                   EXHIBIT 10.51

                               COMMERCIAL SUBLEASE

1.   PARTIES: PSC Management Limited Partnership ("SUBLESSOR") agrees to lease
     to Perot Services Company, LLC ("SUBLESSEE") the property described in
     Section 2 below.

2.   PROPERTY: A portion of the property described as Lot 1 Block A, Atlantic
     Richfield Subdivision and located at 2300 West Plano Parkway, City of
     Plano, in Collin County, Texas (the "PROJECT"), such portion being more
     particularly described on the attached EXHIBIT A, together with any
     improvements, furniture, fixtures, telephone equipment and any other
     property located thereon. Sublessee may expand the property subleased
     hereby to include the portion of the Project more particularly described on
     the attached EXHIBIT B, together with any improvements, furniture,
     fixtures, telephone equipment and any other property located thereon, at
     the same rental rate per rentable square foot, by giving Sublessor at least
     90 days prior written notice of such expansion. All property subleased by
     this Sublease is hereinafter referred to as the "PROPERTY".

3.   AMENITIES: Sublessee shall be entitled to (i) the use of one reserved,
     secured parking space and 50 reserved, covered parking spaces located at
     the Project at locations identified on the attached EXHIBIT C; (ii) secured
     filing and storage space for employees of Sublessee at a location at the
     Project mutually satisfactory to Sublessor and Sublessee; (iii) use of all
     facilities at the Project (including without limitation, lobbies,
     pedestrian ways, public corridors, the cafeteria, fitness center, jogging
     track and other common areas) on the same basis as employees of Perot
     Systems Corporation ("PSC"); and (iv) use of the board room and theater at
     the Project, subject to coordinating scheduling with PSC (it being agreed
     that PSC shall have first rights with respect to the board room and
     theater).

4.   TERM: This Sublease is effective on the date hereof. However, the term of
     Sublessee's occupancy begins on SEPTEMBER 15, 2002 (or such earlier date as
     Sublessee shall occupy the Property) and ends on APRIL 30, 2005 (subject to
     earlier termination as hereinafter provided). Sublessee shall use the
     Property only for general office purposes. So long as Sublessee is not then
     in default hereunder, and provided that Sublessor has either purchased the
     Property or the Overlease (hereinafter defined) is renewed for a period at
     least equal to the period of such renewal term, Sublessee may extend the
     term of this Sublease for an additional TWO years at the same rental rate
     per rentable square foot by giving written notice of such extension to
     Sublessor at least 120 days prior to the expiration of the primary term. In
     addition, Sublessee may terminate this Sublease for any reason upon 120
     days prior written notice from Sublessee to Sublessor and payment to
     Sublessor of the unamortized portion of the Allowance (hereinafter
     defined), calculated on a straight-line basis over the initial term of this
     Sublease.

5.   RENTAL: During the term of this Sublease, Sublessee agrees to pay
     Sublessor, without offset or demand, at such place as Sublessor shall
     designate, a monthly amount equal to one-twelfth of $15.50 per rentable
     square foot of the Property, prorated for the first month of the term of
     this Sublease. Rent shall be due and payable on SEPTEMBER 15, 2002 and
     thereafter on the first day of each month during the term of this Sublease.
     The rentable square footage of the Project is deemed to be 729,107 square
     feet. The rentable square footage of the property described in EXHIBIT A is
     deemed to be 23,424 square feet and the rentable square footage of the
     property described in EXHIBIT B is deemed to be 1,546 square feet.
     Sublessee shall pay to Sublessor, as additional rent, the cost of
     electricity used at the Property. Such calculation shall be determined by
     allocating to the Sublessee its Proportionate Share (hereinafter defined)
     of the electrical costs attributable to the Project. Such payment shall be
     due thirty days after written invoice. Such Proportionate Share shall be a
     fraction, the numerator of which is the gross square footage of the
     Property and the denominator of which is 729,107. Notwithstanding the
     foregoing, so long as H. Ross Perot, Sr. ("HRP"), is chairman of the



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     board of directors of PSC, no rent or electricity costs shall be payable by
     Sublessee with respect to the 2,784 rentable square feet of the Property
     used as the chairman's office.

6.   BROKERS COMMISSION: Sublessor shall pay $10,000 to Capstar Commercial Real
     Estate Services, Inc. for consulting services relating to this Commercial
     Sublease.

7.   SECURITY DEPOSIT:  None.

8.   ALTERATIONS/REPAIRS: Sublessor shall remodel the Property in accordance
     with plans and specifications and using a contractor approved by Sublessee
     and Sublessor. Sublessor shall pay an amount equal to $4.00 per rentable
     square foot of the original Property (the "ALLOWANCE") as an allowance
     toward remodeling costs of the Property. Sublessee will promptly pay the
     costs of remodeling in excess of the Allowance. Sublessee shall be
     responsible, at its own expense, for installation, operation and
     maintenance of all servers and LANS related directly to Sublessee's tenancy
     and will coordinate with Sublessor regarding all servers, LANS and other
     communications and data installations. Subject to SECTION 13 below,
     Sublessor agrees to promptly repair at its expense the roof, foundation,
     HVAC, life safety, sprinkler, elevators, electrical, gas, plumbing and
     other mechanical systems of the building and exterior walls only, excluding
     all windows and doors, upon the receipt of written notice from Sublessee
     requesting repairs. Sublessee shall take good care and maintain at its
     expense the remainder of Property, and upon the termination of this
     Sublease deliver the Property in good repair and condition, reasonable wear
     and tear and damage by fire only excepted. Sublessee shall not make any
     material alterations, additions or improvements to the Property without the
     written permission of Sublessor. All such additions and fixtures (except
     trade fixtures) shall remain and become the property of Sublessor, unless
     Sublessor requests their removal, in which event Sublessee shall remove
     same and restore the Property to its original condition at Sublessee's
     expense. Sublessee shall keep the Property in a neat and clean condition.

9.   FIRE: In the event the Property, or a portion thereof, shall be damaged by
     fire, or other casualty insurable under standard fire and extended coverage
     insurance, and neither Sublessor nor Sublessee elects to terminate this
     Sublease as provided below, Sublessor shall proceed to rebuild and repair
     at its expense. If the Property shall (a) be substantially damaged by a
     casualty not covered by Sublessor's insurance, (b) be rendered untenantable
     in excess of five percent of the floor area by a casualty covered by
     Sublessor's insurance or (c) suffer damage to the extent that the remaining
     term of this Sublease is not sufficient to amortize the cost of
     reconstruction, then Sublessor or Sublessee may elect to terminate this
     Sublease by giving written notice to the other within 15 days of the date
     of such casualty. Sublessee shall not permit the Property to be occupied
     for any purpose deemed illegal, disreputable or extra hazardous on account
     of fire, nor permit any actions that will increase the fire insurance rate
     on the Property. Sublessee shall not bring or store hazardous substances on
     the Property.

10.  EMINENT DOMAIN: If any of the parking spaces subleased hereby or more than
     one percent of the floor area of the Property, or such portion thereof as
     will make any portion of the Property unusable for the purposes herein
     leased, shall be taken by law, ordinance or regulation for public use, this
     Sublease, at the option of either Sublessor or Sublessee, shall terminate
     effective the date possession is taken by the condemning authority, and
     rental prorated. All compensation awarded for taking of the Property shall
     belong to Sublessor. Any award to Sublessee for loss of business or
     personal property shall belong to Sublessee. Neither party shall have any
     right to any award to the other by any condemning authority.

11.  ASSIGNMENT: Sublessee shall not assign, sublet, mortgage or pledge this
     Sublease, nor permit the whole or any part of the Property to be occupied
     by others without the written consent of Sublessor;



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     provided, however, Sublessee, without Sublessor's consent, may sublet all
     or part of the Property to entities directly or indirectly controlled by
     HRP or members of his family, subject to the terms and conditions of this
     Sublease.

12.  LAWS/USE: Sublessee agrees, at Sublessee's expense, to comply with all
     governmental laws, rules and orders relating to Sublessee's use and
     occupancy of the Property. In no event shall Sublessee allow operations at
     the Property to constitute a nuisance to the other tenants of the Project.

13.  INDEMNITY: Sublessee agrees to keep the Property covered with liability
     insurance in amounts, from underwriters and in a form reasonably acceptable
     to Sublessor at the cost of Sublessee and to indemnify Sublessor and hold
     it harmless from any loss, expenses or claims arising out of the use of the
     Property by Sublessee, its employees, invitees, agents or visitors or any
     other person whatsoever. Sublessor shall not be liable for any injury or
     loss on or about the Property to Sublessee, its agents, invitees,
     subtenants, licensees or concessionaires or any other person entering the
     Property. Sublessor shall not be liable to Sublessee for any injury to
     person or damage to property caused by defect or failure of equipment,
     pipes, wiring, broken glass, backing up of drains or by water, gas,
     electricity or oil leaking or by any portion of the Property becoming out
     of repair. Sublessee shall keep all of its merchandise, fixtures,
     equipment, leasehold improvements and other personal property insured by
     fire and all risk replacement cost insurance. Sublessee's insurance shall
     be primary.

14.  ENTRY: Sublessor or its representatives shall have the right to enter the
     Property during normal business hours upon 24 hour written notice to
     inspect, make repairs or alterations to adjacent property or show the
     Property to prospective purchasers, lessees or lenders. Sublessor shall
     have the right to enter the Property at any time without notice if
     necessary to protect persons or property from immediate threat of injury or
     damage. Sublessee shall not be entitled to abatement of the rent by reason
     thereof.

15.  SIGNS: Except with the Sublessor's prior written permission, Sublessee
     shall not place any signs or objects on the roof or any portion of the
     exterior of the Property; make any changes to or paint the exterior;
     install any exterior lighting, paintings, signs or displays; or place any
     sign or display on fences, sidewalks, parking lots or driveways of any type
     that may be viewed from the exterior of the Property. Use of the roof above
     the Property is reserved to Sublessor.

16.  DEFAULT: The following events shall be deemed to be a default by Sublessee:
     1) failure to pay any installment of rent and the continuation of such
     failure for at least ten days after written notice to Sublessee or 2)
     failure to comply with any provision of this Sublease, other than the
     payment of rent and the continuation of such failure for at least 15 days
     after written notice is sent to Sublessee. Upon the occurrence of either of
     the above, Sublessor may pursue any remedy available at law or in equity.

17.  LIENS: Sublessor waives its statutory landlord's lien on the personal
     property of Sublessee.

18.  TAXES: Sublessor is responsible for rendering and paying real estate taxes
     on the Property. Sublessee is responsible for rendering and paying all
     personal property taxes on Sublessee's personal property, trade fixtures
     and inventory placed on the Property.

19.  UTILITIES/TELECOMMUNICATIONS/JANITORIAL SERVICES: Except as provided
     herein, Sublessor agrees to provide without additional charge all utilities
     used on the Property (including light bulb replacement) and to provide
     janitorial services considered standard by Sublessor. Sublessee shall have
     the right to interview and approve personnel used by Sublessor for
     janitorial services, such approval not to be unreasonably withheld.
     Sublessor shall bill Sublessee certain telecommunications costs (including
     without limitation, the cost of certain multi-line handsets,



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     specific use circuits, long distance fees and maintenance) pursuant to a
     separate agreement. Internet connections will be provided pursuant to a
     separate agreement. Sublessee shall pay for its cost of telephone and
     security service.

20.  SEVERABILITY/SUBLEASE: In the event a court declares any part of this
     Sublease invalid, the remaining portion shall remain in full force and
     effect. Sublessee hereunder agrees that this Sublease is subject and
     subordinate to the lease under which the Sublessor hereunder occupies the
     Property (the "OVERLEASE," and the lessor and its successors in interest
     thereunder being herein called the "OVERLANDLORD") and that Sublessee
     hereunder will, upon the request of the Overlandlord (or its Lender),
     attorn to the Overlandlord (or its Lender) and pay the Overlandlord (or its
     Lender) all of the rents and other monies required to be paid by the
     Sublessee under this Sublease, and perform all of the terms, covenants,
     conditions and obligations contained in this Sublease; provided that, as a
     condition to and if Overlandlord (or its Lender) requests such attornment
     by Sublessee, the Overlandlord (and if its Lender requires such attornment,
     then also such Lender) shall agree to recognize Sublessee hereunder, and
     this Sublease shall continue as a direct lease between Sublessee and
     Overlandlord (or such Lender, its successors or assigns, if it or such
     successor or assign becomes the owner of the Property) upon all of the
     terms and conditions hereof; and provided, further that in no event shall
     Overlandlord (or any successor owner of the Property) have any obligation
     to perform any obligation of the Sublessor hereunder with respect to
     obligations of the Sublessor accruing prior to the date that Overlandlord
     (or such successor owner of the Property) becomes the Sublessor hereunder,
     and that any obligations of Overlandlord (or such successor owner of the
     Property) hereunder arising after such date shall be without recourse to
     Overlandlord (or such successor owner of the Property), other than the
     interest of Overlandlord (or such successor owner of the Property) in the
     property demised by this Sublease. The term "LENDER" shall initially mean
     the holder of any mortgage or deed of trust or similar lien against the
     Property from Overlandlord or its predecessor in interest, but upon any
     judicial or non-judicial foreclosure sale or deed in lieu thereof, "LENDER"
     shall mean the purchaser at such sale or transferee by deed in lieu
     thereof. A termination of the Overlease shall cause a termination of this
     Commercial Sublease.

21.  RISK OF LOSS: Sublessee assumes all risk of loss to items that it places on
     the Property. Sublessor bears no risk of loss for Sublessee's property.

22.  NOTICES: All notices required or permitted herein must be in writing and
     may be delivered in person, or by mailing to the addresses shown herein, or
     to the address of the Property for Sublessee, by certified mail, return
     receipt requested. Such notices shall be deemed to be delivered as of the
     date of posting, whether actually received or not. This Sublease contains
     the entire agreement between the parties hereto, and no agreements,
     inducements or promises, oral or otherwise, not a part of this agreement,
     shall be binding on the parties hereto.

23.  WAIVER OF SUBROGATION: Anything in this Sublease to the contrary
     notwithstanding, Sublessor and Sublessee hereby waive and release each
     other of and from any and all rights of recovery, claim, action or cause of
     action, against each other, their agents, officers and employees, for any
     loss or damage that may occur to the Property, or merchandise, fixtures,
     equipment, leasehold improvements and other personal property within the
     Property (or within any storage areas), by reason of casualty loss, fire or
     the elements regardless of cause of origin, INCLUDING NEGLIGENCE OF
     SUBLESSOR OR SUBLESSEE AND THEIR AGENTS, OFFICERS AND EMPLOYEES, but only
     to the extent that such claims are covered by insurance or, by the terms of
     this Sublease, are required to be insured. Because this section will
     preclude the assignment of any claim mentioned in it by way of subrogation
     or otherwise, each party agrees immediately to give to each insurance
     company which has issued to it policies of insurance coverage relating to
     the Property, written notice of the terms of the mutual waivers contained
     in this section, and to have the insurance



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     policies properly endorsed, if necessary, to prevent the invalidation of
     the insurance coverages by reason of the mutual waivers contained in this
     section. THIS SECTION RELEASES A PARTY FOR ITS OWN NEGLIGENCE.

     EXECUTED in multiple originals this the _________ day of
__________________, 2002.

<Table>
<Caption>
     SUBLESSOR:                                               SUBLESSEE:
     ---------                                                ---------
<S>                                                           <C>
     PSC Management Limited Partnership,                      Perot Services Company, LLC,
     a Texas limited partnership                              a Texas limited liability company

     By: PSC GP Corporation, a Delaware                       By:
         corporation, its general partner                         ------------------------------
                                                              Its:
         By:                                                       -----------------------------
             ------------------------------------
         Its:
              -----------------------------------

     Address:     2300 West Plano Parkway                     Address: 12377 Merit Drive, #1700
     -------      Plano, Texas 75075                          -------  Dallas, Texas 75251
</Table>


Exhibit A: - Leased Premises
Exhibit B: - Expansion Premises
Exhibit C: - Parking Location



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