I used to have Norton (2006) and got rid of it last year after getting fed up with the performance issues (the computer, not me.)
I tried the trial versions if AVG and others and none of them seemed any better...in fact a couple were worse! The PC kept slowing down and it was rendererd unusuable a lot of the time.
In despair, I returned to Norton as at least I knew what it was like....the 2008 version of Internet Security seems completely different now! I turned off all the automatic updates (I run them myself when I have time) and it runs quite happily in the background without doing anything I don't tell it to. Even the boot up is a lot quicker - it used to take about 5 minutes before now it is just the same as it was with no AV software installed.