I like AVG a lot, but they have made it insanely hard to find the free version: you have to find the relatively tiny link on each of several pages trumpeting the benefits of the pay versions. It's the Diet Coke of evil.

So here is a direct link. (http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html) (It's not on avg.com but it is where they send you.)