If you still can’t solve it, then download the latest stable version of Avast from the official website, AND the Clean Uninstall Utility from http://www.avast.com/uninstall-utility, reboot into Windows Safe Mode and run the uninstall utility following the instructions.

Then restart into Normal Mode to use the new latest stable version you just downloaded, and reboot again.

If that doesn’t resolve the problem, then the problem can’t be resolved from Avast side, and you should look for some Windows-related solution (or some theme-related tool).