Being behind a router is one of the best ways to protect your PC, especially if you have more then one PC hooked up to it.
A good program for under $50.00 (US/CAN) is Trend Micro I have been using this for 3 yrs and have never had any problems, (won't work with Zone Alarm though). It has POP/SMTP scanning, Spyware scanning and a real time emergancy disconnect from the internet when it detects a network virus.

I agree with the statements about Norton, any time my company purchases those stupid OEM's (Compaq's) I immediatley remove the Norton software, which is a chore.

But if you are after the free versions, then ZA or Sygate is plenty sufficient for home use.

IF you have Win Xp (with all of the updates) their firewall (not the best) should be adequate behind a router.