a timer for the PC - screen tme for the kids

Before I develop this, I thought it may already exist, and the classiccmp mail list might be the place to ask. What we have, is the screen time problem with the kids. If we are not there hounding and policing them, they will be on for hours. All the medical community says, we need to limit their screen time, as it contributes to their AD disorder and schoolwork, homework failures. My idea was initially do this in hardware, with a timer, and a solid state relay to gate the AC to the PC. On further thought, I should be able to do this in software, with a timer that lets the PC run for an hour, and then shuts the PC down until the next 24 hour cycle. (Installs itself on windows startup) Has anybody seen this, before I re-invent the wheel?