Personally, I have AVG set to scan each day at 8am, and if I'm awake and using the computer at that time it gets switched off. I check the test logs once in a blue moon because I'm confident that I woun't get anything, and my confidence is justified by my track record. If I did get something I'd know - either my the misbehaviour of Windows, or from my firewall...
...and as Nodsu said, I have real time scanning anyway, and can scan downloaded files etc from the context menu if I need to.