It seems like Avast is preventing the game from starting if you are using some of the recent experimental builds.
You might need to update Avast or use the following workaround provided by a reddit user:
Fix that worked for me: (On mobile so this might not be exactly correct) Go into the realtime protection section of avast, then settings, and find the exclusions section. Add the whole folder that Cata is in, click apply and ok. Then it should work like new!
If you have this error, make sure you manually check for an avast update.
It did not notify me, but a program update was available upon a manual check, and solved the issue completely.
For me and another user in the issues tracker, the problem was apparently caused by a minor glitch in a previous update that caused Avast to isolate the .exe files for scanning, but never actually scan and un-quarantine them.
Ah, interesting. As I’ve reported elsewhere, the typical problem is Avast wanting to Derpscreen Deepscreen each new build of Cataclysm. If you get impatient (due to the screen not appearing for a decent amount of time) and try to open it a second time, you’re pretty much fucked until you restart.
Since then I did get an update for Avast and the last couple times I grabbed a new build, it didn’t bother Deepscreening hem. So this likely has been fixed, barring any other undiscovered lunacy.
As Avast has the highest ratings under free. I still get my feathers ruffled by them being to money grubbing lately. They simply stopped caring after version 7 =(