Help: 32-bit or 64-bit?

How do I know whether my version of Windows is 32-bit or 64-bit?

Windows Vista

Click Start
Right-click Computer
Choose Properties from the context menu
In the "System" section look for "System Type". It will say either "32-bit Operating System" or "64-bit Operating System"

Windows XP

Click Start
Right-click My Computer
Choose Properties from the context menu
Right under the words "Microsoft Windows XP" will be the the Edition of XP you're running, such as "Professional x64 Edition" or "Home." If that line contains the word "x64" then you are running a 64-bit edition of Windows XP.