I had this same issue.

I ran the windows 8.1 update on 10/19/2013 and installed it, then today I got a notification popup that windows defender had turned on because avast was unable to start.

So I looked for Avast and it wasn’t listed in my list of programs and the desktop shortcut for it was gone.

I downloaded the 2014 version of Avast and installed that and had to restart my system and it seems to be functioning properly now.