Perhaps your operating system is set to shut down your machine every hour, or some sort of parental controls are set? To test if this is the case, you can boot your computer into "Safe Mode with Networking" and use that for an hour or longer. If your computer does not shut down after an hour, then it's safe to assume it's something in Windows. If safe mode also restarts, then perhaps it also affects safe mode or is a hardware issue.
If you have another operating system, perhaps you could boot into that to test. You could burn Ubtuntu to a DVD disc for example, and try (not install) Ubuntu for over an hour (Ubuntu will let you try it's OS without installing on your hard drive, it runs completely off the disc.) If your computer still restarts with Ubtuntu, then most likely a hardware issue or your BIOS has been set to restart after a time period.
If you're willing to test Ubuntu, please let me know how it works out.