Norton is OK as far as protection, as what I have seen in the 2007 edition, however it is still quite heavy on system resources. AVG free or Avast! is better on system resources, and at picking a few more things. If you're going to use a free Anti-Virus program, head towards AVG or Avast!, or Avira, they work very well for free applications. If you want to go buy one for yourself, something like NOD32 or Bitdefender is a lot better.
As you'll notice, the applications you pay for will usually offer frequent updates, customer service, and a higher level of sensitivity when scanning in comparison to free programs. Personally, I'm using AVG Free at the moment, and it works like a charm. Other people will reccomend the program they are using, simply off experience, but either way you will be happy with which ever one you pick.