Why do that? 32 bit programs work perfectly fine under a 64 bit OS. If, after a upgrade from say XP 32 bit to Win7 64 bit, you have a program that does not work, the problem is that some older programs that worked in XP simply won't work in Win7. There is a XP mode in Win7 you can try to run the program in.
In short, if the program won't run in Win7 64 bit, then it very most likely won't run under the 32 bit version either.