The difference usually is that the free ones may have a few less options and/or nag screens to try to get you buy the better versions.
Personal preference most certainly comes into the equation but for configuration options I've always liked AntiVir PE as it has all the important features like automatic updates, real time scanner that runs in the background all the time watching for issues, and it is still far less resource hungry than McAfee not to mention that I've never run into any conflicts or problems with it which is far more than most McAfee users can say.
The only down side to AntiVir PE is the nag screen and I even found a fix for that on the internet which prevents it from running so I am a happy camper and it doesn't cost me anything in money or stress.
































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