How long has it stayed in 'Shutting Down' mode?
The ultimate resolution to this problem, though not ideal, is to hold the power button down for 10 seconds. That should get the PC to shut down. When you boot back up, you may get the 'Windows didn't shut down properly' message. Choose 'Start Normally' if that's the case. If you boot to Windows fine, try a restart and see if the shutdown problem occurs again.