imho this was probably a rights issue - some virus's change the ownership of the file so even admin users can't see the file, you have to turn off simple filesharing to even start to see this occuring. once you've turned off simple files sharing you can get into the advanced properties of the file and you can adjust its security and ownership (different things) of files.

Obviously AVG has the ability to take ownership of files so it can check them for virus's

thats my theory anyway

personally what you should be doing now is backing up all your data and do a fdisk of your system (delete the partition, recreate, format of your HD) and reinstall your system.

While you've removed the virus, its already done (potentially) its damage and opened up a hole in your system for the virus writer to get control of your pc.

if you do any online banking - change your passwords