Warning: Make sure that there aren't any important downloads or documents running at the time of activation. If there are, tell the victim how to cancel it (explained at the end of the instructable).
NOTE: Will only work with Vista.
NOTENOTE: I will NEVER clean up my desktop! :D
Step 1: Prank settings
In the window that pops up, type in:
shutdown.exe -s -t 120 -c "Your computer has committed suicide, have a nice day."
The number behind the -c is the time in seconds that the computer will shut down in, you can put in any amount here. If there isn't anything important going on, select a short time such as 5 seconds or so. If there IS something important, set it to a longer time such as 120 seconds or more so that you have time to deactivate it.
In between the quotation marks, write a message such as 'WARNING: VIRUS DETECTED: BEGIN SHUTDOWN IN 60 SECONDS'.