It's more than likely doing that due to a data or driver error so right-click the My Computer icon, select Properties, then the Advanced tab. Click the Startup and Recovery button and remove the check box next to Automatically Reboot. When the machine restarts next, it will display a blue screen (a STOP error) with something like "IRQL_NOT_LESS_THAN_EQUAL". Tell us what the error is and the name of the file mentioned and we'll see if we can pin point the prob for ya.
Also could be the PSU so what rating is it? :smokin: