BlueScreenView is a free program requiring no installation which lets you view a history of your Windows installs' Blue Screen of Deaths (BSOD).  Since most BSODs give long incomprehensible codes for their causes, and there are no utilities available while they are showing to copy and paste them, this tool is invaluable for troubleshooting.

Why do BSODs happen?  Most often it is hardware related.  Faulty RAM is the culprit I have apprehended more than any other.  Overheating components are next.  Misbehaving drivers are third on the list.  Last, but not least, are the myriad random causes that only Google can help solve.

So what do you do when you get a BSOD?  I generally ignore it the first time I see it.  The next time it comes up I perform a google search for the "Technical Information" presented at the bottom of the screen.  Somewhere in the first page of results somebody else has brought up this exact same issue and somebody else has solved it (in a discussion board that has been indexed by Google).