Norton Antivirus 2005 has a significantly better detection rate than any free antivirus, according to the tests I've seen at places like
http://www.av-comparatives.org.
BTW if you want to get the most out of it, go through the configuration and ensure that the heuristic detection is enabled and maxed, and that it is scanning within compressed files on the real-time scanner as well as the scheduled scans. It can detect adware/spyware and other non-viral Bad Stuff too, so enable those options if you want extra protection there.
You can get daily Intelligent Updater updates from
this page to stay more up-to-date than the default weekly auto-updates, and at the rate viruses come out these days, that might be a good habit