What's a good FREE anti-virus program for Windows 7?

I’ve loaded AVG 8.5 onto Windows 7 (build 7100). I’ve seen good reviews for it before so I thought it would be a good choice. However, it doesn’t seem to be 100% compatible with Windows 7. The little "flag" on the tray says that AVG is not reporting its status correctly to the Windows Security [...] http://bit.ly/p1rdMf