When I was making a living fixing people's pc's, I unfailingly took norton off and replaced it with antivir personal edition. There were several times when I would get a sick pc in with fully up-to-date norton. On a few of these occasions, I did a full virus scan with norton, then disabled or uninstalled norton, and did the same with antivir. Antivir found a few viruses that norton completely missed.
This is all completely aside from the fact that norton is incredible bloatware, capable of chewing up memory and cpu cycles at an astounding rate, while providing inferior protection to boot! If you want to slow your computer down while generating lots of heat inside your computer case, norton is an excellent start.