I haven't used it for several years, so not sure if it's still a great recommendation (or what the specifics of the license agreement are), but AVG used to be the best of the free options:
http://free.avg.com
[edit] Hm, looks like the free version is licensed for personal, non-commercial use only. And, of course, they don't give you much in the way of details on their non-profit licenses:
http://www.avg.com/ca-en/avg-antivir...nonprofit-home