Open Control Panel - System - Advanced
In the Startup and Recovery section (bottom) click on the Settings button
to open the Startup and Recover window.
In the System Failure (middle) section of that window there are 3
checkboxes, the bottom one labelled Automatically restart. Click on that
checkbox to clear it.
Click on OK and Apply as needed to exit.
The computer will no longer restart when a system
failure occurs. Instead it will probably throw up a "Blue Screen Of
Death" with a STOP error message that you can read and research.
You can also type "eventvwr" into run and open the logs - look for the critical errors.