I had a *huge* problem with AVG about a year ago, when a virus took it over. My computer was basically slowly disintegrating from this virus, and AVG wasn't doing anything. I didn't know until my computer decided to restart over 20 times in an hour..when it recovered I installed and ran another virus scanner and found that AVG had been infected and my computer was in a terrible state.
Since then I use "eTrust Antivirus" (
http://www.my-etrust.com/) which is also what they use here at MicroSoft. If its good enough for them, its good enough for me.
I also run monthly scans using housecall as well.