I don't know honestly norton's slowed my computer way down but might as well use it since it's free cause dad got three licences. Avast has found alot of stuff nortons didn't and avg free not that great antvr only tried because it was free.
I voted avast, i have avast pro and it only takes up about 4 mb memory, sometimes can take up to 20mb, this includes how much virtual ram its using too, it still catches any type of malware/virus, avast is the best! ;D
McAfee Internet Security sucks, it came free with this laptop with a 90 day subscription, i got rid of it after a week of using it, it made the computer soooo slooow, and missed a lot of viruses, it also didn't let me use msn because the firewall was messed up