Try a repair…Control Panel>Uninstall a Program>double click “avast”>click “repair”>reboot

If that fails try a clean install download and save the avast uninstall tool from here.
Run uninstaller. You will be directed to reboot into safe mode…click “yes”.
Reboot
Re-install avast.
Reboot and register or insert license.

The reason I asked you to do so much was to eliminate as many possibles as we could.
You will find Revo is not that good and most if not all of us don’t recommend its use.
The one good thing is that you no longer use MSE. Microsoft doesn’t even recommend its use.
See http://www.howtogeek.com/173291/goodbye-microsoft-security-essentials-microsoft-now-recommends-you-use-a-third-party-antivirus/