try the InitiateSystemShutdown API call:

BOOL InitiateSystemShutdown(
LPTSTR lpMachineName, // pointer to name of computer to shut down
LPTSTR lpMessage, // pointer to message to display in
// dialog box
DWORD dwTimeout, // time to display dialog box
BOOL bForceAppsClosed, // force applications closed flag
BOOL bRebootAfterShutdown // reboot flag
);


Get the definition from the API text viewer.
There is more detailed documentation in the MSDN.

Matt