I dun believe in free anti virus, why save the few bucks and have the hassle of virus infection than u need to reformat / reinstall your pc.
i used norton security below is some feedback on other brand i used b4 (just my 2 cents worth)

mcafee - reformat my pc once becos of virus attack using mcafee, virus disable my mcafee try to scan / update virus but failed
trendmicro - one of the best at detecting virus but cannot kill them
kaspersky - search & destroy, found & kill virus, be careful it may delete impt files too
panda - cheap only SGD$7.90 per yr, still on testing
norton anti virus - used it for 2 year no problem

agreed with H4XF4XTOR surfing habit plays an impt part but accidents do happen like clicked on wrong stuff or downloaded infected files if you can spend a few thousand to set up your machine why save on the anti virus.