ok heres a computer prank thats completly harmless and funny at the same time.
its called the shutdown prank
right click on desktop click new shortcut erase the target coding and put this into it C:\WINDOWS\system32\shutdown.exe -s -t "10" -c "hard drive overload is in effect" s stands for seconeds
t stands for time
and c stands for comment
press next
make the icon anything you wont, my fav is internet explorer
all this does is when your freind or whoever clicks on the icon you create it shuts down the computer 100% harmless
heres another but ill leave it to youtube to explain it
YouTube - Computer Hack - Make PC Appear Frozen