i have had my pc for 5 years with no anti virus and i have never gotten a virus hehe
I reccomend, if you want a free, on system virus scannr, the AVG virus scanner. Good at picking up viruses, scans email, and other stuff. Best of all, it updates on its own whenever they add definitions. Its for the cheap consumer.

http://www.grisoft.com/us/us_index.php