Originally posted here by The Duck
I also have to say Avast! is, IMHO, the best free AV I have ever used. It's installed on all of my computers except my gaming computer (the less running tasks, the better) and I also install it on most of the computers I service, unless the owner is stubborn and prefers Norton...
Avast! is pure garbage. I had it running on a workstation that I use for a lot of testing purposes and virus research. Just tonight I ran a full system scan and it detected three infected files. I removed Avast! and immediately installed McAfee Enterprise 8.0i. I kicked off another full system scan with McAfee and it detected 72 infected files. This was with the latest virus sigs for both products.

Just part of the list Avast! didn't detect:

W32/Loosky.gen@mm
W32/Banwor.worm.dll
Proxy.Raser trojan
Backdoor-CIU
QLowZones-4 trojan
Spam-Maxy trojan

I have no confidence whatsoever in Avast!'s detection capabilities. During my research, the on-access scan set to its highest levels failed to detect hardly anything. I wouldn't recommend Avast! to anyone.