Document:

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                                                                    EXHIBIT 10.6

                                AMENDMENT NO. 2
                                     TO THE
                           NCI BUILDING SYSTEMS, INC.
                         NONQUALIFIED STOCK OPTION PLAN

                                November 4, 1999

         The NCI Nonqualified Stock Option Plan, as amended and restated as of
December 12, 1996 and as amended by Amendment No. 1 to the NCI Building
Systems, Inc. Nonqualified Stock Option Plan, dated March 4, 1998 (the "Plan"),
is hereby amended as follows:

         1.       Section 18(g) of the Plan is hereby restated in its entirety
         as follows:

                  "(g) "Normal Retirement Age" means the age established by the
                  Board from time to time as the normal age for retirement of a
                  director or employee, as applicable. In the absence of a
                  determination by the Board, the Normal Retirement Age for all
                  participants shall be deemed to be 65 years of age."

         Except as amended by this Amendment No. 2, the Plan shall continue in
full force and effect as originally executed and delivered. Any reference in
the Plan to the "Plan" shall refer to the Plan as amended by this Amendment No.
2. All capitalized terms used herein and not otherwise defined shall have the
meanings assigned to those terms in the Plan.

         Signed to be effective the date first written above.

                                      /s/ Donnie R. Humphries
                                      -----------------------
                                      Donnie R. Humphries, Secretary<PAGE>   1
                                                                     EXHIBIT 4.1

<TABLE>
<S>                                     <C>                                                         <C>
               ----------------                                                                      ----------------
                    Number                                 [PROBEX LOGO]                                  Shares

               ----------------                                                                      ----------------

     This certificate is transferable in
     Portland, OR and New York, NY

                                                            PROBEX CORP.
                                        INCORPORATED UNDER THE LAWS OF THE STATE OF COLORADO

                                                                                                             CUSIP 742670 20 1
          THIS CERTIFIES that
                                                                                                                SEE REVERSE SIDE
                                                                                                             FOR CERTAIN DEFINITIONS

          is the owner of

                        FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, NO PAR VALUE PER SHARE, OF

                                                            PROBEX CORP.

          transferable on the books of the Corporation by the holder hereof in person or by duly authorized attorney
          upon surrender of this certificate properly endorsed. This certificate is not valid unless countersigned by
          the Transfer Agent and registered by the Registrar.
               WITNESS the facsimile seal of the Corporation and the facsimile signatures of its duly authorized
          officers.

                              /s/ THOMAS G. PLASKETT       [PROBEX CORP. SEAL]             /s/ BRUCE A. HALL
                    PRESIDENT AND CHIEF EXECUTIVE OFFICER                                     SECRETARY

                                                   Registered and Countersigned:
                                                          TRANSFER ONLINE
                                           227 SW Pine St., Suite 300, Portland, OR 97204

                                       by
                                          --------------------------------------------------
                                                         Authorized Officer
</TABLE>
<PAGE>   2

                            [PROBEX CORP. LOGO]

<TABLE>
<S>                                                                  <C>

     The Corporation will furnish without charge to each stockholder who so requests the powers, designations, preferences and
relative, participating, optional, or other special rights of each class of stock or series thereof and the qualifications,
limitations or restrictions of such preferences and/or rights. Such requests shall be made to the Corporation's Secretary at the
principal office of the Corporation.

     KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN, OR DESTROYED THE CORPORATION WILL REQUIRE A BOND OF INDEMNITY AS
A CONDITION TO THE ISSUANCE OF A REPLACEMENT CERTIFICATE.

     The following abbreviations, when used in the inscription on the face of this certificate, shall be construed as though they
were written out in full according to applicable laws or regulations:

     TEN COM   --   as tenants in common                              UNIF GIFT MIN ACT -- ________________ Custodian ______________
     TEN ENT   --   as tenants by the entireties                                               (Cust.)                   (Minor)
     JT TEN    --   as joint tenants with right of                                         under Uniform Gifts to Minors
                    survivorship and not as tenants                                        Act _____________________________________
                    in common                                                                                 (State)

                                                                       UNIF TRF MIN ACT -- ___________ Custodian (until age ______),
                                                                                             (Cust.)
                                                                                           _________________ under Uniform Transfers
                                                                                                (Minor)
                                                                                           to Minors Act ___________________________
                                                                                                                  (State)

                                 Additional abbreviations may also  be used though not in the above list.

          FOR VALUE RECEIVED, ______________________________________________________ hereby sell, assign and transfer unto

  PLEASE INSERT SOCIAL SECURITY OR OTHER
      IDENTIFYING NUMBER OF ASSIGNEE
-------------------------------------------

-------------------------------------------

------------------------------------------------------------------------------------------------------------------------------------
                           (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

------------------------------------------------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                              Shares
-----------------------------------------------------------------------------------------------------------------------------
of the common stock represented by the within Certificate, and do hereby irrevocably constitute and appoint
                                                                                                                            Attorney
---------------------------------------------------------------------------------------------------------------------------
to transfer the said stock on the books of the within named Corporation with full power of substitution in the premises.

DATED
      ---------------------------------

                                                            X
                                                             -----------------------------------------------------------------------
Signature(s) Guaranteed
                                                            X
                                                             -----------------------------------------------------------------------
                                                     NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME(S) AS
                                                             WRITTEN UPON THE FACE OF THE CERTIFICATE IN EVERY PARTICULAR, WITHOUT
                                                             ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATEVER.

By -----------------------------------------------------------------------

THE SIGNATURE(S) MUST BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS,
STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP IN
AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO S.E.C. RULE
17Ad-15.

</TABLE><PAGE>   1
                                                                     EXHIBIT 4.2

--------------------------------------------------------------------------------

                                  PROBEX CORP.
              INCORPORATED UNDER THE LAWS OF THE STATE OF COLORADO
                         AUTHORIZED 1,000,000 SHARES OF
              10% CUMULATIVE CONVERTIBLE PREFERRED STOCK, SERIES A
         NO PAR VALUE PER SHARE, $10.00 LIQUIDATION PREFERENCE PER SHARE

THIS CERTIFIES that

is the owner of

     SHARES OF THE 10% CUMULATIVE CONVERTIBLE PREFERRED STOCK, SERIES A OF

----------------------------------PROBEX CORP.----------------------------------

transferable only on the books of the Corporation by the holder hereof in person
or by attorney upon surrender of this certificate properly endorsed.

     In WITNESS whereof, the said corporation has caused this certificate to
be signed by its duly authorized officers and its corporate seal to be hereunto
affixed this      day of          , A.D., 2000

/s/ THOMAS G. PLASKETT                                 /s/ BRUCE A. HALL
-------------------------           [SEAL]             ---------------------
      PRESIDENT                                              SECRETARY

                         Registered and Countersigned:
                                 TRANSFER ONLINE
                 227 SW Pine St. Suite 300, Portland, OR 97204

                            by
                              --------------------
                              Authorized Officer

                NOT VALID UNLESS COUNTERSIGNED BY TRANSFER AGENT

--------------------------------------------------------------------------------
<PAGE>   2

                                  PROBEX CORP.

<TABLE>
<S>                                                                  <C>
     A full statement of the rights, preferences, privileges and restrictions granted to or imposed upon the respective classes and
series of shares authorized to be issued and upon the holders thereof is set forth in the articles of incorporation of the
Corporation, as amended and supplemented, on file in the office of the Secretary of State of Colorado. A copy of the statement of
rights, preferences, privileges and restrictions may be obtained upon request and without charge from the Corporation at its
principal executive office.

     Each share of the 10% Cumulative Convertible Preferred Stock, Series A is convertible into 5.33333 shares of common stock
subject to adjustment.

     The following abbreviations, when used in the description on the face of this certificate, shall be construed as though they
were written out in full according to applicable laws or regulations:

     TEN COM   --   as tenants in common                              UNIF GIFT MIN ACT -- ________________ Custodian ______________
     TEN ENT   --   as tenants by the entireties                                               (Cust)                   (Minor)
     JT TEN    --   as joint tenants with right of                                         under Uniform Gifts to Minors
                    survivorship and not as tenants                                        Act _____________________________________
                    in common                                                                                 (State)

                                                                       UNIF TRF MIN ACT -- ___________ Custodian (until age ______),
                                                                                             (Cust)
                                                                                          _________________ under Uniform Transfers

                                                                                           to Minors Act ___________________________
                                                                                                                  (State)

                                 Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED, ______________________________________________________ hereby sell, assign and transfer unto

  PLEASE INSERT SOCIAL SECURITY OR OTHER
      IDENTIFYING NUMBER OF ASSIGNEE
-------------------------------------------

-------------------------------------------

------------------------------------------------------------------------------------------------------------------------------------
                           (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

------------------------------------------------------------------------------------------------------------------------------------

------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                              Shares
-----------------------------------------------------------------------------------------------------------------------------
of the capital stock represented by the within Certificate, and do hereby irrevocably constitute and appoint
                                                                                                                            Attorney
---------------------------------------------------------------------------------------------------------------------------
to transfer the said stock on the books of the within named Corporation with full power of substitution in the premises.

Dated
      ---------------------------------

                                                            X
                                                             -----------------------------------------------------------------------

                                                            X
                                                             -----------------------------------------------------------------------
                                                             NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND WITH THE
                                                             NAME(S) AS WRITTEN UPON THE FACE OF THE CERTIFICATE IN EVERY
                                                             PARTICULAR, WITHOUT ALTERATION OR ENLARGEMENT, OR ANY CHANGE WHATEVER.

Signature(s) Guaranteed
                        --------------------------------------------------------

By
    ----------------------------------------------------------------------------
THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION
(BANKS, STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH
MEMBERSHIP IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT TO
S.E.C. RULE 17Ad-15.

                              ELECTION TO CONVERT
                        (To be executed upon conversion)

    The undersigned hereby irrevocably elects to convert __________ shares of 10% Cumulative Convertible Preferred Stock, Series A
represented by the within Certificate into _______ shares of Common Stock.

    Please issue a certificate or certificates for such shares of Common Stock, in name of, and pay any cash for an fractional
share to:

    PLEASE INSERT SOCIAL SECURITY OR
OTHER IDENTIFYING NUMBER OF ASSIGNEE
---------------------------------------

---------------------------------------

Name
     -----------------------------------------------------------------------------------------------------------------------------

Address
        --------------------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------------------------

Signature Guarantee                                              Signature
                                                                           -------------------------------------------------------
                                                                           NOTE: THE ABOVE SIGNATURE SHOULD CORRESPOND EXACTLY
------------------------------------------------------------               WITH THE NAME ON THE FACE OF THIS CERTIFICATE OR WITH
                                                                           THE NAME OF ASSIGNEE APPEARING IN THE ASSIGNMENT
                                                                           FORM ABOVE.
</TABLE>

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