CPUOff is a totally free application that is continually watching the workload of the CPU, and is able to activate the automatic shut down when the loading reaches a set level over a set period of time.
Both the CPU loading and many other parameters can be configured in the configuration window. The CPUOff program can also shut down another computer remotely. The shut down is designed to force closure before all the applications are up and running.
NOTE: CPUOff needs an activation code to be used, which you can get for free from the author`s webpage.