Cause of spontaneous restart?

[Monty]

My machine shuts down and restarts occasionally when a new
application is launched or when I open a minimized window.
What is the cause of this?

 

[Ray at]

Spontaneous reboots are typically caused by hardware issues. If you've
noticed a pattern that this happens when you're repainting your display (by
maximizing an application, as you say, for example), I'd try replacing the
video card to see if that helps.

Ray at work