This is definitely a hardware issue. If you have recently added hardware, remove it and reboot. If not, then I would remove all PCI, USB, and IEEE 1394 devices and start again. Add a device until you have a problem. In addition, if you have the problems with nothing installed, you may have a problem with the Motherboard, processor, RAM, or power supply.

Secondly, check for conflicting IRQs. You may need to move devices around on PCI slots (farther from original the better). Windows XP and 2K does not natively support IRQ changes with the ACPI HAL. If your computer automatically shuts itself off when you choose shut down from the menu, then it does. If it comes up with a screen that advises it is safe to shut off your computer, it does not.

I hope this helps a little.