I have also been having the same problem on Windows 7, since an update a few weeks ago. I temporarily removed Avast and installed AVG, which shuts down the system with no problem. I have now reinstalled Avast from scratch, and it still won't shut down the system.
Is a solution to this problem likely to be found, or will the problem simply be ignored?
Is it possible to download an earlier version of Avast to work around the problem?