You must have Windows 7 Professional, Enterprise, or Ultimate to run Windows XP Mode.
However, as Cryptodan has already said, most of the 32-bit programs can run quite well in Windows 7 64-bit editions.
There are some exceptions though. 32-bit programs which require ring-0 level access, programs which need 32-bit device drivers (like printer software), programs which hook other system applications will not run in 64-bit Windows.
Edited by Romeo29, 15 November 2010 - 10:26 PM.