Go into Control Panel, System Properties, and click the Advanced tab. Uncheck the box under Automatically Reboot. Make sure that you have Write An Event To The System Log checked.
The next time it reboots it's self, go into Computer Management, select System Tools, select Event Viewer, and select System Errors Log. If it is a driver or software conflict, you should be able to see what it was. I think if you have the "Automatically Reboot" box unchecked, it will give you a blue screen instead of rebooting it's self. If it does that, it is more than likely a driver/software conflict. If not, it is more than likely the RAM , power supply, or something hardware related.
EDIT: Before you wait for it to reboot again, I think that the "Write an Event to the System Log" is enabled by default. Just go into the System Error Log right now and look at the log instead of waiting for it to reboot it's self again, and look back through it to see if you can find what gave it problems the last time.