Personally, I would stick it out with Norton untill your subscription wears out. I'm not going to say that I like it or anything but if you've paid for it you may as well get what you paid for. When your subscription wears out then I'd go with AVG Free. If you are looking to pay for another subscription I'd go with BitDefender.
One note is that the Add/Remove Programs applet in windows XP (Programs and Features in Vista) will likely leave remnants of Norton behind so when it comes time to remove it, make sure you go to
Norton Redirect - Symantec Corp. and run the Norton removal tool, otherwise you're going to have to manually remove registry entries.