Just to back up other posters:
There is absolutely need to pay for internet security products. As has been suggested, AVG Free is a great anti-virus solution. (It automatically updates itself every day, so your PC is always fully protected from the latest threats). They've recently 'shot themselves in the foot' by sending out messages that version 7.1 will no longer be supported from next January without mentioning that the latest version, 7.5, will continue to be free and fully supported. (i.e. 1960darrenh's error is understandable but Ethel's post is correct).
That's your anti-virus solution sorted out. All you really need on top of that is a decent firewall. As has been suggested, Zone Alarm Free is excellent:
http://download.zonelabs.com/bin/media/images/ butt/download_trial.gif
Those two programs provide a complete solution to genuine 'security' problems. Adware and spyware aren't real security threats but they can slow your PC down. Ethel seems to have gone 'over the top' with her collection of programs. Ad-aware SE Personal is probably sufficient:
http://www.lavasoftusa.com/products/ad-aware_s e_personal.php
Chris