If your systems is rebooting all by itself, there is a real good chance that you have a hardware problem.
Windows will sense it and reboot in an attempt to avoid damage to the system. Problem with that is you cant locate the problem cause it avoids the blue screen of death which will give u a location of the problem.
you can shut the reboot off to get a blue screen. Wont fix anything but it will point to the problem at least.
start -> Control Panel -> System -> Advanced tab -> the Settings button in the Startup/Recovery section -> uncheck Automatically Restart option
your video card, indicated by a driver called nv4_disp.dll. Another potential suspect is your sound card, alcxwdm.sys. Before you try any of the following, see what comes up in your BSOD.
If you still get reboots after you shut off windows reboot, then you more than likely have a bad power supply.