All versions of XP Home, XP Media Center, and XP Tablet are 32-bit. There is a 64-bit version of XP Professional, but it never came preinstalled on computers. If you are running XP, you are almost definitely running 32-bit; however, if you are still unsure, go to Control Panel -> System. If you are running 64-bit XP Pro, it will say so under the “System” heading.
Windows Vista or Windows 7
Step 1: Go to the Start Menu, right-click Computer, and select Properties.