There are several reports (for example https://forum.avast.com/index.php?topic=160717.0) that a the recent Microsoft KB3000850 update for Windows 8.1 is incompatibe with Avast and results in all kinds of problems (for example, see http://forums.cnet.com/7723-33042_102-630845/windows-8-recent-updates-creat-havoc/?messageId=5631669#message5631669).
If this happens with you: uninstall the update and hide it to prevent automatic re-installation. To hide it: in IE go to Tools>Windows update, let the site propose to download it, then right click on it and choose 'hide'.
Since it's an optional update, it won't harm to leave it that way for a while, until the issue is solved by Avast. Then unhide it again. https://support.microsoft.com/kb/3000850 tells about its contents.
Kees

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