Dont pay a penny. I use the following. These are all highly recommended by 1000s of people. Other people will suggest alternative programs all do the same job. Personally I have tried pretty well all of them and these are the ones I advise for those who's PC's I fix and maintain.
Zonealarm: a firewall to keep the nasties at bay. Update only when you feel like it. It doesn't change how the program works.
AVG Free Antivirus: To remove and detect any nasties that get through. Let it run every day in the background.
Spyware Blaster: To prevent spyware getting on the PC (update once a fortnight). Nothing to run here.
Spybot Search and Destroy: To remove the spyware that gets though, run a scan each week and update beforehand.
As I said just try suggested programs and pick what you like. Personally I prefer these ones above. Plus I have linked to them, so makes life easy.