If you have a 64-bit CPU, you might be itching to try a 64-bit OS. While software support has grown from Windows XP x64 Edition, you'll probably still want to keep Windows XP 32-bit on your hard disk as a backup. PCSTATS has written a short but sweet guide on installing multiple versions of Windows, which shows you just how easy it is.
"To most people, the logical answer is to install Windows XP x64 Edition while still keeping XP 32-bit intact on the system. This allows you to mix and match the two operating systems, avoiding software failure and discovering which is best for each application and game. While this is a bit of a drain on hard drive space, it's more than worth it in terms of frustration avoided. It's also easy to do, and in this short guide, PCSTATS is going to show you how to go about creating your 64-bit/32-bit XP dual installation."