I'd have to say AVG is the worst of the free scanners. I have tested several on VM's that got viruses from torrenting (like, random infections from hackers attacking it) My results: Avira, stopped the viruses, left behind a lot of garbage. Realtime scan did not catch the viruses, but a full scan did.
Avast, Stopped the viruses but only with full scan, realtime scan did not catch them
AVG, did not stop the virus, did not detect the virus until it tried to propagate itself into system restore. Failed to quarantine or delete the files once they were in system restore. Full scan did not detect the viruses anywhere.
Symantic, did not detect the viruses
Panda online, did not detect the viruses.
Tested in 2011.
So in summary, they all failed to detect the virus in real time. 3 managed to detect the virus in a full scan, only 2 actually stopped the virus. In my testing, I forgot to check if Avast actually cleaned up the garbage the virus left behind.
So I would recommend either Avira or Avast, although latest Avira has gotten almost viral itself, making hidden administrator users and changing file system permissions on it's own files, and locking out other admin users, and it's nagware has gotten really irritating lately. I have Avira on my XP desktop at the moment, I plan to trade it out for Avast in the future. --- He who stumbles around in darkness with a stick is blind. But he who... sticks out in darkness... is... fluorescent! - Brother Silence