Mine's getting flaky, too. I'm getting sore tempted to wipe out all the cookies, but then I have to re-log in on my sites.
I just use Microsoft Security Essentials (MSE) on the theory that MS is best equipped to keep their own system (W7 Home Premium) working, and has access to all the latest definitions the other AV sites have discovered.
That's the theory, anyhow.
But I wonder if somehow some evildoer might have buggered up the MS download of MSE definitions such that it won't detect his own evilware.
Had Norton, which came with the HP laptop's HP pain-in-the-ass overlay, but holy crap, it was constantly popping up with warnings, so I disabled it.
Maybe I shouldn't've.
Terry