Stims

Stims runs subtly in the tray and can prevent the computer shutting down due to inactivity for a selectable period from 30 minutes to 2 days.

It can also be locked on. This program can be set to simply prevent shutdowns, or it can make the computer think someone is active by simulating a button press every minute. Stims can also be activated via the command line, and can be set to auto-start with Windows and/or automatically close when the Stims run out.