To be honest, both suck. While they are pretty decent considering they are free, in most cases there are lots of false alarms. I've been using Eset smart security for a while (somebody i know has the business versions, gave me a free copy) and it's pretty awesome but i've reinstalled windows more times than i can count, so i just didn't bother installing.. running w/o anti-virus for months now.. if i get a virus i'll just reformat and reinstall. But back on topic i would say AVG is better, Avast sucks and i've tried both and found it to be one of the worst i've used by far, and the Microsoft one is also pretty decent.