Open System Properties via Start > Control Panel > Performance and Maintenance > System
[ System Properties may also be opened using the WinKey+Pause key combination ]
Select the Advanced tab and then click Settings in the Startup and Recovery section
In System Failure section, clear the checkbox next to Automatically Restart
Click OK and OK to exit
Next time it crashes instead of shuuting down and restarting it will give you an error message, Post the exact message here.