AVG, Avast and Microsoft Essential are all pretty good. You should also add spyware and firewall software, both of which you can get for free at download.com (it's a safe site...run by CNET).
If you want to have full versions, look for Fry's ads. They often run specials on security software for $20 or less. Just punch in the name of the company into google for ratings.
Finally, security software is absolutely no defense for stupidity/naivety. Don't open any e-mails/attachment from people you don't know. Heck, don't open attachment from people you know unless you know exactly what it is. Don't go to site you're not sure about or click on deals/links that seem too good to be true.