Since Microsoft announced its own anti-malware product, Microsoft Security Essenstials, back in late 2008, the product has been steadily getting better. When asked what I use for anti-virus, I always answer MSE or AVG Free.
Both cost my favorite price, free, and are well regarded in the security community.
The big news of course, is that MSE is now free to small businesses with 10 or less PCs. I've actually been recommending it to smaller businesses for a while and now they'll be in compliance. (Now if only it was okay for servers...)