When adding a Server 2008 R2 domain controller a 32 bit 2003 server the command “adprep /forestPrep” needs to be run on the 32 bit server. The adprep program comes on your 2008 R2 DVD but when you run this on your old 32 bit server you get an error:
The image file C:\adprep\adprep.exe is valid, but is for a machine type other than the current machine. This is because your trying to run the 64 bit version not the 32 bit version.
I finally found that I already had a 32 bit version of adprep. The location is dvdrom:\support\adprep\adprep32.exe on your Server 2008 DVD!
adprep32 /forestPrep worked like a charm.