How To Get Back At A Certain Smart Alec Who Unplugged Your Hard Drive

Step 6: The result

Picture of The result
The victim's computer will boot as normal until they have logged on.  After they log on, it will freeze on the "welcome" for a few minutes, then it will display a black screen.  I'm not sure if this is the same on every Windows system, but no matter what, the system will not boot.