had avira, but their ads are pretty unacceptable and annoying(and the ways to remove them didn't work lately), now i have avast, and the only complaint so far is the frequency at which it reminds of an update(a bigger one, not just data).
as far as protection goes i heard both of them are somewhat decent, but i'm no expert here.
i've heard some good things about ms security essentials, but then again, dunno anything more and i have heard good things about IE too...