Best Way to Detect Computer Left On?

No, the AT scheduler wouldn't give you the ability to stop the shutdown once it started, but you could kill the scheduled shutdown before it starts if you were using the PC.

Then again, if you use different user accounts, you can set a local or domain-level (or AD) security profile so that it enforces account or system usages times... I'm not sure you can force a shutdown this way, though. And it would depend on the OS as to how granular this would be.

Either hook into a screensaver timeout, or use the power management to shut the machine down or go into sleep mode after so many minutes of inactivity. Or use a scheduler that shuts it down at a specific time.

No one solution is going to cover every scenario, though. Maybe the real solution is to teach the kids to do it right or suffer the consequences.

Here are some possibilities:

The one I use: - lots of good features, including sending keystrokes to running programs, checking if a program is already running, etc.

The one already in XP:
On a similar note, I have two PC's on one APC UPS unit. PC2 is running the Power Chute (APC's UPS monitoring) software and can shutdown automatically after being on backup for a time I designate.

Question is, how can I get PC1 to shut down as it does not have PowerChute running on it. I was thinking to have PC1 be the HomeSeer computer, then rig the alarm in the UPS so I get a contact closure to my Ocelot, then run the shutdown script above (triggered Event via the contact closure).

Any other less arduous suggestions?
There is a version of PowerChute that allows networked machines to receive a shutdown notice from a master UPS. I use it here for my machines and at one time I had a tiered shutdown system based on it.

The product is called PowerChute Network Shutdown, and when I got it (few years ago), it was free for downloading on the APC site. However, I have used it only in conjunction with an SNMP- and web-based management card in a master UPS (client-server). I'm not sure it'll work with a locally-attached UPS and allow remote machines to be notified as well. APC does have a ton of systems and software and I'm sure somehow you can get a system working for you.
FWIW, PH has a distributed component that can execute files on remote machines. That, plus something like Wizmo should do what you need.