I'm not sure, but afaik "shutdown.exe" can shutdown machines over the network.
Google for it. However, I've never tried this method and I'm unsure if it even works.
What will probably work is shutdowing a system through remote control software like
Radmin or VNC, as you can simply go to the start menu and select shutdown (though in
Radmin you can also use a special function to shut the machine down without actually
going through Windows)