I know this thread is a few months old but I just wanted to add how I fixed my avast problem (can't enable shields). My problem started today and after a couple hours of trying to figure it out (multiple cleans/fixes uninstalls reinstalls 20 reboots), I went into add/remove programs. There are 2 programs there "xna 3.1" and "xna 4.0". I uninstalled xna 4.0, rebooted, and my avast works just fine now. Hope that might help anyone having the same problem.