A lot of people say that Microsoft knows it's own operating systems better than anybody else and they have a greater interest in protecting those systems. Therefore, Microsoft Security Essentials was optimized for those operating systems. Other folks say that MSE is too basic and AVs like Avast provide more (better) user options and protection.
IMHO it's the computer user who poses the greatest threat to the computer - not the AV suite that's installed. If someone insists on going to adult sites, downloading questionable files, haphazardly using torrents, etc., the computer will eventually become infected regardless of which AV suite is installed.