Norton Antivirus is also on my black list. I had several computers which it seemed make unstable. They would hang on reboot, spontaneously crash if left working for more than a day, etc.
The problems had the hallmarks of some kind of resource leak, but I never could find out what was leaking. I couldn't pin the problems directly to NAV, except that they went away when I installed AVG.
Plus, because I sometimes disinfect severely infected computers, from kids, etc, I was able to do some comparisons, and Norton was not finding everything.
You're right. It's very resource intensive and it's not as up to date as it should be.