I got sick of AVG Free not picking up things. I ended up switching to NOD32 (not free). I heard good things about avast. There's also Avira AntiVir which has been getting good ratings from PC Magazines. http://www.free-av.com/ it's free and you can also update to a paid version if you really wanted to.
I had avg free on my last computer. It was brutal. Before that I had F-Prot and loved it, then they stopped supporting the version I had.
I used to use AVG until I tried to uninstall it one time. It caused errors every time I loaded Outlook and it was impossible to remove it completely. I'm an Avast fan now. It's very good, and lightweight.