+2, clean installation is the only way to move from 64-bit to 32-bit. Why do you want to do this? Just as a fair warning, a program designed specifcally for 64-bit windows, will not work on a 32-bit system...
Take a look at these FAQs before making the move - http://windows.microsoft.com/en-us/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions
Microsoft Windows Outreach