avast free version for years, silent mode and boot time scans are great if ever i do something dumb or i need to clear someone else's harddrive.
AVG is good too but i had a couple misses with that when i used it
don't recommend Norton, the free version is almost worse than useless and even the paid for version isn't any better than the free versions of others (had loads of conflict issues with drivers when i used it too although it may be better by now)
life's one big game
windows vista ultimate
i5 @3.3ghz, 4gb ram, geforce gtx460, directx 11