The latest issue of Consumer Reports has a discussion of anti-virus software (free and pay). Between free and pay there isn't any significant difference in performance between the four free programs and the better pay programs. The main thing that pay programs offer are complete packages including firewall and anti-spam. Since you can get free firewall and anti-spam separately, there seems to be no advantage to buying anything. The four free packages studied (in rating order) were Avira, AVG, Avast, and Microsoft security essentials. However the differences were quite small.