Take a screenshot of the computer screen with all its icons... then hide the icons and make the screenshot the new wallpaper... then watch the victim start up their computer and click on the 'icons' on the wallpaper, then they restart and the process repeats - this goes on for a while and then they call up IT services and scream at some unsuspecting support tech...

BTW I had assumed I would be the only one who knew of 'Ummagumma' here- another great track to use to scare the living crap out of some one is found at the end of one of a 'guess Who' song...its at the end of a song I think is called 'Hang on to your life' The lead singer reads a passage from Revalations in a creepy voice... Its after a pause of maybe 2 minutes of silence, so if youre listening to it while drifting off to sleep for the first time you wake up REAL fast reconsidering your atheistic views momentarily lol