public static extern int WTSShutdownSystem(IntPtr ServerHandle, long ShutdownFlags);

  Private Shared Function WTSShutdownSystem(ByVal hServer As IntPtr, ByVal ShutdownFlags As Long) As Integer

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/termserv/termserv/wtsshutdownsystem.asp

public static extern int WTSShutdownSystem(IntPtr ServerHandle, long ShutdownFlags);

    WTSShutdownSystem(new IntPtr(0), WTS_WSD_REBOOT);