I also used to be able to recommend AVG. I used it for about 3 years or so, and it worked fine, but slowly got worse and worse. Sure, it kept my comp clean, but every version got slower and slower. Up to 8.0 was ok, but it was getting so full of crap that it was just slowing me down if anything.
I never actually had it not "let me" do anything, but the way it was going, it's not surprising.
I hate programs that take over and slow things down when there's no reason for it. Also, with AVG, there were about 6 processes running constantly and when it started to get more and more intrusive/bloated, I had to change.
MSE is great for what I need it for, since I don't do a whole lot of "odd things". I haven't had a virus in a long time, so i'm still not sure how good MSE or AVG are at catching things.
If I wanted my comp slowed down for no reason, i'd use Norton or McAffee. Apparently AVG is getting to be that way now too. Too bad, because it used to be pretty unnoticeable while it was running.