I have same problem.
I am running Windows 8.1 Pro. Installed Win Update KB3000850 and after restart upon update installation I had all sort of problems, like: cannot open Control Panel, cannot open some files, and computer was so slow that was impossible to work. I have noticed that I did not have avast icon in the notification area upon restart, as well. Shut down did not turn off computer and restart did not worked at all.
After few queries on google I have uninstalled Avast and computer start working normally again. Later, I have reinstalled IS and free version for few times. I have disabled few options during installation like: NG, Secure DNS, Safe zone and some other and that did not resolve anything, computer was sluggish all the time with avast installed.
I have removed avast completely. Now, computer working like a charm.
When we can expect the fix from Avast?