Here is Microsoft solution on how to determine if your computer is a 32 or a 64 bit Operating System.
The 3 OS they list in this page are Vista, XP, and Server 2003. It will tell you a few steps and you will be able to clearly determine which one it is. I tested it and was able to see right away that I had a 32 bit system because on the System Properties window in Control Panel, it said "Microsoft Windows XP" under System. If no mention of "x64" then it's a 32 bit. Or if it's a Vista system then it's an 86 bit also known as 32 bit.