Have you tried this:
If you haven't changed any of the system failure settings, you should be able to see the error by looking in the Event Log. But a better long-term solution is to turn off the automatic reboot so you can actually see the error when it happens—chances are it will tell you enough about itself to let you troubleshoot further. To change the recovery settings to disable automatic rebooting:
Right-click My Computer, and then click Properties.
Click the Advanced tab.
Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.
Clear the Automatically restart check box, and click OK the necessary number of times.
Restart your computer for the settings to take effect.
Now a fatal error occurs won't cause an automatic reboot and you can sort out what's causing the problem