A guy at work told me about a brilliant one he did to a bloke he used to work with. He took a screen snap of the guy's Windows desktop, set it as his Windows background and then removed all of the icons on the guy's desktop.
The end result was the guy sitting at his desk for minutes, clicking on icons that only existed as a static image and wondering why it wasn't working.
reminds me of a prank i played at work to a complete jack leg,i waited for him to got to the bathroom then got to work!
first I would change his close-down sounds on his desktop to porn sounds(woman orgasm's), turn up the speaker volume to max,
then I would flip his desktop [windows allows you to do this with ctrl+alt+ down]
then i disable shortcut keys, so the only way it would to reset the desktop was to reboot....all that was left was to wait for the comedy to ensue.
now we worked in a call center so he usually answered the phone before logging back into his computer, after he logged in and saw the upside down desktop, it was enough to get him flustered,
first he was gestured the equivalent of "hurr hurr" you flipped my desktop as he spoke to the customer. [i smiled and nodded and tried to contain my laughter for i knew how this was about to play out]
so he desperately tried to reset the computer, leaving the customer thinking he is an incompetent ass and he doesn't know how to use a computer.
Then he would see the shortcut keys were not working then proceeded to reset the computer (he was clearly agitated at this stage).
Funnily enough when the porn sounds started at full blast he freaked out!, the customer thought he was watching porn as did the entire office....
needless to say the computer would not shut down until the computer was finished playing the sounds, by time he got to the speakers it was too late ....note if you work in an office with individual computers LOCK YOUR WORK STATIONS!