I know that when Windows is shutting down, it sends a WM_QUERYENDSESSION message to each application. This makes it easy to detect when Windows is shutting down. However, is it possible to know if the computer going to power-off or is it going to restart after Windows has shutdown.
I am not particularly hopeful, considering the documentation at MSDN has this to say about WM_QUERYENDSESSION: "...it is not possible to determine which event is occurring," but the cumulative cleverness of stackoverflow never ceases to amaze me.
From here:
You can read the DWORD value from "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shutdown Setting" to determine what the user last selected from the Shut Down dialog.
A bit of a roundabout solution, but it should do the trick.