You can specify what your computer will do when you push the power button on your case (hibernate, suspend, ...) but 'Reboot is not one of them... Try looking at the Power Options in Control Panel, page Advanced.
OR
Look @ the Power Options in your BIOS.
OR
Try the Microsoft Knowledge Base... I tried to look it up for you but I get an error on any query I type.
The page you were looking for is currently not available. The address may not be correct, or there may be a temporary problem with this site.
Greetz
Lukas