Determining Inactivity in Windows?

[Red Dorakeen]

Hi, I was wondering if anyone knows a way of determining a period of inactivity in windows (similar to the idea behind having a screen saver activate after a certain period of inactivity). I'm trying to enable a sleep function for my software, to turn off certain devices, but have no idea how to go about determining if/how long the program has been inactive. Any ideas would be a great help!

Thanks,
Red