Chances are if you don't know whether your OS is 64-bit or not then it probably isn't. For me, the Windows XP boot screen displays the "Windows XP x64 Edition" logo as it loads. If yours just says "Windows XP Professional" then it is 32-bit. There are advantages and disadvantages to having a 64-bit OS. It is faster but there are occasional incompatibility issues that I can find to be quite annoying, particularly with video codecs and Windows Media Player. Some videos have no sound and others may not even play at all. I got my 64-bit copy for free from a friend that worked with IT professionals, but I don't think they sell 64-bit XP in stores anymore. You can just check by going to Control Panel > Performance and Maintenance > System and it will say your exact OS, RAM, processor, all the basics. If it doesn't say 64-bit after Windows XP Pro then it is 32-bit. In my personal opinion right now 32-bit is still the way to go though. While programs that are specifically designed to run 64-bit are faster, there aren't too many of them out there right now, and for what you lose with all the incompatibility issues, it's not worth it quite yet. Dolphin still has much improvement to be done in terms of bugs and rendering efficiency even with the 64-bit version, so don't feel like you're being left out--at least for now. Hopefully as 64-bit becomes more mainstream, the incompatibility issues will be worked out, and Dolphin will be able to run most games with little trouble.
