Over the last 15 years I've always made it a point to use only licensed anti-virus software. I've used the best McAfee, Symantec and, lately, BitDefender products. All full-featured versions. And yet an occasional virus, trojan or malware still manages to slip through. You can just imagine the amount of infestation you'll get with the free, limited-featured versions. Nowadays the only sure way to keep your computer virus-free is to never turn it on.