How to simulate Windows shutdown for debugging?

There is a tool named Restart Manager (rmtool.exe) in the Microsoft's Logo Testing Tools for Windows, which can be used to send shutdown and restart messages to a process. Logo testing tools can be downloaded here:

http://download.microsoft.com/download/d/2/5/d2522ce4-a441-459d-8302-be8f3321823c/LogoToolsv1.0.msi

Then you can simulate shutdown for your process:

rmtool.exe -p [PID] -S

where [PID] is the process ID. According to the Vista Logo Certification Test Cases document,

Restart Manager shutdown messages are:

a. WM_QUERYENDSESSION with LPARAM = ENDSESSION_CLOSEAPP(0x1): GUI applications must respond (TRUE) immediately to prepare for a restart.

b. WM_ENDSESSION with LPARAM = ENDSESSION_CLOSEAPP(0x1): The application must shutdown within 5 seconds (20 seconds for services).

c. CTRL_SHUTDOWN_EVENT: Console applications must shutdown immediately.

I believe when Windows is shutting down it sends a "WM_QueryEndSession" to all applications. To simulate a Windows shutdown you could create a little application that just does a PostMessage with this message to your application and see what happens. Windows may send more messages than that to actually close your application (like WM_CLOSE), but whenever your application receives the "WM_QueryEndSession" message it means your application is about to have the rug pulled out from under it.