Windows 2008 R2 is 64bit only. However the Windows Server 2008 R2 includes a 32-bit and 64-bit version of Adprep.exe. If you want to run one of the Adprep.exe commands on a 32-bit computer, use the 32-bit version of Adprep.exe (Adprep32.exe). The 64-bit version runs by default.

To extend the forest schema the administrator should run the following command from Windows 2008 R2 disk on SBS 2003 or Windows 2003 STD

<CD drive>:\SUPPORT\ADPREP> Adprep32.exe /forestPrep

To extend the domain schema the administrator should run the following command from Windows 2008 R2 disk on SBS 2003 or Windows 2003 STD

<CD drive>:\SUPPORT\ADPREP> Adprep32.exe /domainPrep

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Reference:

http://technet.microsoft.com/en-us/library/cc772074(WS.10).aspx