Shutdown XP If no one is logged on

I work in a corporate environment, and we have an in house application that is sometimes left running over night (rightfully so) while it is running processes. However we also have users that simply log off their computers, and leave them running all night. Out of 400 PCs, we have approximately 100-150 that are left on overnight and weekends for no reason with no one logged on. We have another 50-75 that are left on to run the application.

Our goal is to reduce power consumption by shutting down or standing by PCs that are not in use. If the internal application is shutdown/paused/etc it can cause severe interruptions to our business processes, so shutting down the PC is not an option if they are logged on. A lot of our users ignore the policy to shutdown the PC at night, so we are looking for other means to do so.

Is there an application/configuration that I could use to do the following:
If a user is logged on then do not shut down. If a user is not logged on, then shutdown or stand by after a certain amount of time (my idea would be 1 hour, but it could be hard coded if there is no other option)

Ideas? Comments? Suggestions?

Thank you in advance,
Dustin

We can't seem to get the support of someone to go around and take notes on who is leaving their PCs on (has to do with 300+ PCs, and the lack of motivation to have someone stay late enough to do so)

Well the power options like "Turn off harddisks after..." and "Shutdown PC After..." do not recognize if a PC is logged onto, which is what I want to manage.

In addition, there are some that are left on some days, and others left on other days, there are not a select group that are left on, it is a matter of what process is being run at the time when they leave.