I've found a few older programs which would run on XP 32-Bit, which will not run on Vista 64-Bit.
I'm not sure that they would run on Vista 32-Bit or not.

I know Vista-64 makes two folders for program files, program files and program Files (x86).
The (x86) is for 32-bit programs, and most of those do run. The ones I've had issues with are quite a bit older.
Birth of the Federation, runs on XP-32 but not Vista-64, but that came out a ways back and lists Windows 95 as a requirement.
Pretty much anything made today, or in the last few years, should run.
Some of the older stuff might not run, if that is an issue.