When I switched from NAV to AVG, AVG immediately picked up 3 trojans NAV had missed. They had gone right through NAV like a seive.
As far as McAffee, my experience is that McAffee is much better than NAV as well.
Now, recently, AVG caught a few viruses but could do nothing with them. They somehome made it on to my computer even when AVG recognized them.
I ran Panda online scanner and it found 2 and healed them. Dunno what they were.
After an update, AVG caught Reg33.exe and quarantined it, which was one of ones it couldn't heal before.
Overall I'm happy with AVG, despite the PROBLEMS updating the program recently.
MORAL of the story is that some virues will get through somewhere, some how.