I have experience with Avast (free version), BitDefender, and now MSE, and I can honestly say that Avast was good, but no better than any of the other freebies out there. BitDefender was not even Windows7 compatible when I decided to upgrade to 7, and given that I had a nearly 7 month license left on it, and that their customer service was absolutely non-supportive, I just dropped it and moved to MSE, and honestly, I've never looked back. Avast, as I said, was good - but it's bloated and the false positives are indicative of a somewhat 'unprofessional' coding. Not saying it's a bad thing, but it bothered me to no end. MSE is clean, streamlined and uses less resources than Avast (I know this), and AVG (from what I've read).