If someone asks me why his system is slow, first question I have asked him generally is : You got Norton Installed and guess what, Most of the time the answer is affirmative. The reason as Paul talked about, It comes pre-installed as OEMs or at least 90 days trials or its still famous .

Regarding Avast, I have tested the latest version 5 and the best thing which happened with the version is the way they changed their interface and made a new one. But It gave me too many false positives and he false positives were of some well known programs setup files directly downloaded from their respective official sources like Internet download manager setup etc. The performance in terms of memory compared to older versions even though its still a bit of memory hungry.

One other thing even though not related directly to any AV. I hate vista and advise anyone of you to switch to either windows XP or Windows 7 . Just a thought though :P