Windows anti-malware?
Last weekend I got sistaur's computer into a reasonably useful state, but in doing so removed an (outdated) anti-virus program or two. Since it is Windows (XP), I suppose there ought to be some sort of anti-malware program. The problem is, I have no idea about that. I haven't had to deal with keeping Windows secure for some time.
So I am looking for recommendations. The anti-{adware,spyware,virus,whatever} should not drag system performance down noticeably, and would ideally be free. Any recommendations?