In my humble opinion, there is probably nothing wrong with the virus scanners in Norton or McAfee. I think the problem is that because they have to keep you on a continual upgrade cycle, they have to continually be percieved to be adding value to their product. They do this by adding bits and pieces like firewalls and real-time adware watchers etc. When they're all loaded up, they sap system resources like there's no tomorrow.
I've been using AVG and ZoneAlarm trouble free for donkeys years. I like them because they're simple, unpretentious and free. In short - KISS, keep it simple stupid! (not you!)