MaximumPC has a great article on the dreaded "Blue screen of death" window that sometimes appears after a sudden crash in Windows. While the message can be an indication of a fatal problem, it also supply you with the information needed to fix the computer. Too often users see the screen and assume that it's only useful to a computer technician -- and then only to make sure the headstone for your dead computer reads correctly. MaximumPC makes it clear that it's far more likely that a recent hardware addition is the cause of the blue screen.
One problem that MaximumPC did not cover is how to handle the situation where the blue screens sails past in a fraction of a second as part of endless loop of failed boots/restarts. My trick with such situations is to use a video camera to video the screen and then replay the recording until I can read the blue screen.