I'm sorry, that title is a bit misleading, but please continue reading.
Because I'm a “computer guy,” people always ask me about what anti-virus software to use, even though I'm avowedly a Mac user and haven't regularly used a PC in almost a decade. What's funny is that the people who ask me those questions are still using the same operating system that I was using all the way back then, Windows XP.
So the thing about protecting your XP box from viruses is:
STOP USING XP.
I mean, seriously. Microsoft is dropping it stone cold dead on April 8, including XP support for my former AV goto Microsoft Security Essentials. There is absolutely no excuse for you to be on XP anymore, even if you're super poor and can't afford a new machine. (In that case, I suggest installing Linux, which is both free as in “free beer” and free as in “freedom”, and is as secure as you want it to be.)
If your IT department requires you to use XP, fire your IT department. If you're neither poor nor have a poor IT department, just pony up for a new PC, install security essentials and be done with it.
If, for some reason I simply wouldn't comprehend, you really insist on keeping XP, you do still need an up-to-date anti-virus, because *I* don't want *your* machine to be turned into a node on a giant spam botnet. I don't really have an official recommendation for what that AV would be, since I would probably just Google it, but Softpedia has a good list of Security Essentials alternatives that you could try.