Or making a program run itself twice...
I did that once. They were not happy...
By far my favorite prank was making a batch file combined with some visual basic to keep the command prompt from showing on screen. The file would run windows media player on a rick-roll file, killed explorer, and monitored and killed any instance of task manager one tried to start. At that point you pretty much had to restart to turn it off.
Unfortunately I could never put it to use, and no longer have it. :/
belugawhale:
Lol
I remember writing the following program and puting it in the Startup folder
Do
msgbox "Close This!" ,4096+16, ""
loop
translate:
a message box that all makes applications not work until the user responds to the message box and the only response you can do is ok and it has a Critical Message icon that loops endlessly