I would agree that NOD32 is probably the best anti-virus program. By my experience AVG is terrible, and Avast is not very good either. I also personally recommend that people stay away with Kaspersky. Back in Kaspersky V5 they added 'protection' against alternate data stream viruses, however they do (did? I think they changed the way the program does it) by writing ADS onto every file that it scanned. Basically adding footnotes to each file which if different after being read back later, were flagged as being a malicious file (Their words
here. Which kind of makes me suspect it's still done this way). However, there was a bug in the program which caused it in rare cases caused mass corruption of files. Without knowing this I downloaded Kaspersky to try it out (as people were recommending it), and later found out that it had corrupted a bunch of my files. Now, they did release a tool to try and recover the corrupted files, but in my mind that doesn't matter. What good is a virus scanner to protect your data if there is a chance that it might shred it?
In my mind the best virus scanners are just that, virus scanners. Not a whole suite which tries to protect your computer from everything. (McAfee, Norton, Kaspersky Internet Security, ect.) These programs tend to try and take control of your computer, and can cause MAJOR problems during updates. I know all three are known for causes connectivity issues in Windows Vista after Windows Updates. In some cases they even can cause Vista to enter reduced functionality mode after updates. If you are very unlucky that means having to call Microsoft Activations to get Vista reactivated after you sort the issue out.
The other thing to remember is no virus scanner is going to be perfect. They will all let viruses through, no exceptions. As shown with copy protection anything written by humans, can be broken by humans. If someone wants to write a virus to beat the current virus scanners there will. You can find many
articles that support this evidence. So there is no sense in using one that will worsen your computer.
If you suspect that a file you downloaded might have a virus in it then make sure you scan it. There are
websites dedicated for scanning files with multiple virus scanning programs for you.
The best security for viruses and trojans is a diligent mind. The best security for worms is a firewall. This will never change.