Have you tried using PSShutdown? http://technet.microsoft.com/en-us/sysinternals/bb897541.aspx.
As long as you know an admin account on the PC, you can shut it down. I've been using it to shutdown PCs when the server's UPS detects a power failure (all systems that should be shutdown are listed in the command file to execute).
I also use PsService to stop and start services on other servers or PCs.