Just a correction. Windows AntiSpyware became Windows Defender, and it requires SP2.
I personally don't like Norton because it hogs system resources like anything. It slows startup tremendously, and while Norton is popular, it's definitions are not the first to be updated.
Then again, overall, I don't like anti-virus programs because I find that viruses are less frequent than spyware, and most anti-virus programs have problems with removing some trojans/worms or the like.