The only real difference between free ones and ones you buy, is that if you have an infection, you will be able to call their support line for assistance. All antivirii do the same thing, and there is no 100% protection, best advice (believe me i've tried over 20 of them) is to check google for reviews on which 'catches more'. One more thing, stay away from 'security centres', they are not needed and will more than likely end up causing you some sort of internet issue, unless you really 'need' that extra protection.