If you don't have an antivirus, or you do, but its crappy, then check out the new one from MS

http://www.microsoft.com/Security_essentials/

Works on XP and up. I have been using it since beta, and I am impressed. It works well, catches more than the likes of AVG and similar, is not super resource intensive, and doesn't annoy you every 5 minutes that you didn't do some "backup" or other stupid thing that has no place in an antivirus product.

I was also a Windows OneCare user for a little while, and this is much better than that was.