Kaspersky all the way.
1. much less intrusive than Norton. You tell it to always disinfect quietly, it will never ever ever bother you, unlike Norton.
2. Norton will always detect keygens & stuff as threats even if they're not (one of the reason its threat database is bulkier, because it doesn't just detect system threats but threats to the "big companies" bank accounts).
3. Kaspersky processes occupy much less system resources that Norton.
4. Heuristic detection way better than all of the other antiviruses combined. That means it will accurately detect viruses that aren't known yet like stuff that just appeared and wasn't yet analyzed by virus labs.
5. The Internet security package from Norton is outrageously overpriced. It has nothing really useful over Kaspersky to justify the extra bucks.
6. The Firewall in the Internet Security package from Kaspersky is brilliant. I dare to say it comes close to the Linux firewalls. In two clicks you can make sure you computer is under lockdown.
My antivirus preferences:
3. NOD32 (the new version, the old one is decent, but not good enough)
4. Norton & McAffee
I used to work for BitDefender, we studied the market