AVG. It's free and takes up WAY less system resources than Norton/McAfee. (Well, that and it works faster/better.)

http://www.grisoft.com

Get AVG Free Editon. They'll send you a serial in your email.. haven't gotten any spam through sneakemail from them, so they're safe. Of course, you don't get all of the goodies that come with Norton/McAfee, but it's JUST an antivirus program and nothing else.