hehehe i put a picture of an error message as a background to fool my parents on the old computer but i changed the text!!!!
it read:
"Windows has detected an incompetent user! to turn idiot detection mode off please press ok."
and it had the ok button too. glory was that it actually worked on both parents and my sister too!!!! HAHAHAHAHAHAHAHA
the other thing like that that we did was to a kid at school who forgot to log off and walked outta class as we came in. we got a screenshot of his desktop then copied and pasted the images of his icons to fill up the blank space. then we made that the back ground and moved all the icons around onto different back ground replications of it... hahaha sux to be him next time he logged on!!!!! never heard what happened to it afterwards though!!!!