As to the timer, what does it output? Meaning for example a logic high or low for a preset period?
The output of the timer is depend on what you will connect to the output relays.
Yeah. That is what I was getting at. Next, you never mentioned but we will assume a Windows based operating system. I would like to know which one?
I'll give you a few ideas to think about. First the concept of placing a signal in parallel with the front panel On button is not a good idea. While this makes for easy start of a system but using the front panel power button to force a shutdown can be a bad thing depending on the OS (Operating System) power settings.
You did mention you had some programing experience so here is what I suggest so you can experiment a little. If you have a system sitting there with a RS 232 port you can use that for some basic functions. Other than the actual data transmit and receive lines (pins 2 & 3) the RS232 serial interface has a few pins that can be quite easily used as DIO (Digital In Out). For example the CTS pin 8, DSR pin 6 and the DCD pin 1 all can be very easily used as digital imputs sending a true (logic high) or False (logic low) to any software you write.
While I am not very good with this stuff I can read those pins using VB (Visual Basic) and have the OS respond to changes like shutdown, logoff or restart. Those pins are low by default and a logic high is as simple as 5 to 9 volts being applied to any of those pins and ground (pin 5). If you want to take that route maybe I can give you a few code sample programs to work with. They would be in VB 6.0.
Also, I would like to see what carbonzit has in mind as he is also active in this thread and had some thoughts.
My understanding is that in Internet Cafe and Kiosk situations that all the workstations are interfaced through a central server computer. That computer controles the time limits. I know that is far from your objective.
Ron