I have 7 PCs. I have had very little trouble with Viruses, Trojans, or Worms.
Norton is a resource hog, and can be difficult to install/uninstall. It also gives a lot of false positives as well s being expensive and not supported for long after the new versions come out.
Mcaffe is about the same in my brief experience with it.
I am now using AVG Free. I have read it is not as effective as others but do not often run into anything anyway.
I have heard good thing about Avast free edition, but have not tried it.
In addition I use Lavasoft AdAware and Spybot free editions. Spybot is not as stable as AdAware, but works well. Whatever one does not catch the other does.
I also run ZoneAlarm free Firewall.