This thread on google answers http://answers.google.com/answers/threadview?id=569374 contains some very good information on the subject. A lot of people will recommend the AVG Free program. It’s good!
This of course comes on the heels of the Windows XP WMF vulnerability crisis. There still isn’t a patch out from Microsoft, and there won’t be for another week. Antivirus companies however (the good ones) had a signature for this problem out a week ago. http://www.securityfocus.com/columnists/377
It goes to show that no matter how patched and safe you try to be with your computer, nothing beats a quality AV program that is updated daily.