dude avg is crappy
infact it has no proper on-access scanner and usually dont detect a lot of trojans until u scan it manually on the file

avast.. the on access is super excellent..
the virus recovery database is freaking fast..
and u dont need it to scan the whole hdd as often as mcafee and avg.
also its not prone to service attacks like avg and nav

best is bit defender.. but i find it .. like fort nox.. it keeps virus out and annoys u a lot of time
kapersky was superb on xp but very buggy for vista