AVG is pants - detection rates are something like 83% compared to 97% plus for paid for AV software such as Kaspersky or F-Secure. AVG is also particularly poor at scan within compressed files such as .iso, .rar, .zip etc
I was running AVG for ages using a email account that received 100s of spam emails each week, many containing attached viruses. AVG nuked the majority of them, with the spam emails forwarded into a junk mail folder. As part of transferring data from one PC to another, I created a backup of all my email folders in a compressed .eba file. On my new machine, I ran scan using Kaspersky on all the transferring data and over 20 viruses were identifed in the .eba which AVG has previous missed, even before they were compressed.
If you really must use a free(ish) AV (which is a false economy IMO), download Norton via Google Pack:-
http://pack.google.com/Incidentally, the likes of PC World sell the complete McAfee package for £25.