haha... well.. I had Vista running Norton 2007. I just shot it and put Avast on. I can't honestly tell how much memory it uses since Vista's task manager is all weird, but it certainly uses less cpu time and doesn't whine every few days about system scans like Norton did. AVG is very good also.
That said, both with Avast and AVG I had instances where one would not see a (known to be) infected file and the other did!! I didn't have Norton then; I wonder if that would have seen it. Obviously all packages have had loads of updates since, so hopefully that little chestnut has been fixed.
Rakku