You have one of a few things occuring.

First, you have a defective hardware device that is plugged into a PCI slot that can also be defective. I would remove all hardware from PCI slots except your video card and see if you still have the problem. I think that you will find this will stop the computer from restarting on its own. After you have accomplished this, then reinstall one device at a time until you find the device that is causing the problem. If not, then you may have a AGP, video card, or RAM problem (most likely not RAM but still a possiblity).

Another possiblity is the sharing of IRQ's. You can check this out by Start, Programs, Accessories, System Tools, System Information, and expanding the hardware section and clicking the IRQ option. You will be able to see if there are any conflicts or too many devices sharing one IRQ. If this is the case, you can usually move the devices that are sharing the IRQ to different PCI slots and this will reassign the device to a new IRQ address. Usually older or less expensive motherboards have these types of problems (your motherboard is not the best on the market *opinion*).

Hope this helps and feel free to post back.