Reboots, Boot loops, System Error, Shutdown, BSOD, Oh My!

Reboots and Bootloops
Computer reboots intermittently and flashes a Blue Screen of Death.
Or, stuck in a constant loop of reboots.
Stop this vicious cycle and find out what is causing your issue:

Disable Automatic Restart after System failure:

XP:
Control Panel (Classic View) -> System -> Advanced Tab.
Click Settings under Startup and Recovery
Uncheck Automatically restart -> OK

VISTA:
Go to Control Panel -> System
Advanced system settings -> Advanced Tab
Cick Settings under Startup and Recovery
Uncheck Automatically Restart under System Failure -> OK. (x2)


System Error, Shutdown, Blue Screen of Death!

Windows encountered a problem and needs to shut down.
Now that you can view the Blue Screen of Death, you will be presented with valuable information on the nature of your system error. The error might identify a driver (a possible driver update may be in order) or a stop error which can be deciphered to unveil the source of the error/bug.
The best resource to identify Stop Errors and their parameters is on MSDN listed under Bug Check Codes. These codes can be used to help troubleshoot your issue.

One Stop Error I received most frequently was 0x0000007B which can have several causes.
A great Knowledge Base Article from Microsoft on the infamous Stop 0x0000007B Error offers some advanced troubleshooting of this common Stop Error.