How to be evil:
Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.
A Help desk guy speaking to a lady user...
Help desk: Double click on "My Computer".
Lady: I can't see your computer...
Help desk: No... click on "My Computer" on your computer.
Lady: How the hell can I click on your computer from my computer?!
Help desk: There is an icon labelled "My Computer" on your computer... double click on it...
Lady: What the hell is your computer doing on my computer?
I changed my password to "incorrect."
So whenever I forget what it is the computer will say "Your password is incorrect."
A wife send her husband an sms on a cold winter evening: "Windows frozen".
The husband send answer back: "Pour some warm water over them".
Some time later husband receives answer from his wife: "Nice going. The computer is completely screwed now."
I found some internet history from my wife on my computer where she'd typed "how to leave husband". It got me really worried.
How did she find out the password to my computer?