That is the default action for a System Failure error.
Open Control Panel, open System go to the Advance tab, click settings under
Startup. In
the System Failure section (middle) click on the checkbox labeled
"automatically restart" to clear it.
Click on OK and Apply as needed to exit. Restart the computer to put the
changed settings into effect.
Now instead of a reboot you will get a "Blue Screen Of Death" error
message with a STOP code identifying the error. From that you may be able
to determine the cause or you can post the error here as we might be able to
figure it out.
If this does not resolve it or the auto-restart option is already unchecked,
please try the following web site:
http://www.aumha.org/a/shtdwnxp.htm