The only thing that I did today before Avast! tuned UNSECURED was that I updated Adobe Flash to latest 10.1 version, but that shouldn't be a reason I think.
It is good practice after doing an install (or uninstall) of any software to
reboot your machine. So perhaps by not doing this, and since you were dealing with Flash, which often times uninstalls prior versions before installing newer versions, caused the instability to Avast.
I always keep an eye on my Avast toolbar and recommend to other to as well. Besides the red "X", you may see a yellow "!" that require you to open the GUI to see what action is required.
But as Vlk pointed out, many of these issues are hopefully being resolved in the next release of 5.1.
