Don't buy anything, just install Microsoft Security Essentials. It's the most lightweight program I've found for this sort of thing, and it gets high marks from every report I've seen on the subject.

I install MSE on all my clients' computers (in addition to my own), and if something ever sneaks through (no antivirus will catch everything), I run Malwarebytes' Antimalware (terrible name, good product). It's also free for the version that doesn't actively scan.

Now, queue the many people on this board who don't think you need antivirus. Oh, and Mark will chime in with a "install Linux."