G'day Jonathan. For me (Win 7 64-bit, avast! free 6.0.1367), it's absolutely a Windows 7 SP1 issue. I installed Windows on a new hard-drive, installed avast!, everything was fine, worked through the Windows updates... same problem as you. I thought something weird must have happened, so I started again. Problem came back once SP1 was installed. Problem went away when I used System Restore to rollback SP1. Problem came back again when I undid the System Restore changes...
I've found two ways to get around it - stop the Web Shield altogether, or add the browser .exe as an excluded process (Expert Settings | Exclusions).
I haven't been able to find a mention anywhere else that it's SP1 that messes with avast! Maybe you could test it by rolling back SP1?