Paid AV these days is generally bloated crap, especially Norton.
Spybot Search & Destroy is a good (the best?) free antispyware app, Avast is a good free antivirus, and Malwarebytes Anti-Malware is very useful if you find yourself with an awkward virus Avast won't remove.
To be honest, it's VERY difficult to just "pick up" a virus these days if you're behind a router and use NoScript when browsing. The only ways you'll get viruses are a) dodgy software/keygens or b) allowing Javascript by default and visiting a suspect site.