I can't remember exactly how to do this one but i'll try and remember cuz it's great. I've been on a mac for the last couple of years and this one is easier to pull off on a PC.
1.) You know how a PC has a power up Tone/music. Well what you do is download a, how can i put this in censored words, "erotic love making sound clip". After you have downloaded this sound clip you upload it onto a USB flash chip.
2.) Now go to a public library/lan cafe, or anywhere where there are multiple public computers and change that computers power on tone/music to your specifically downloaded "erotic love making sound clip".
3.) Once you have done this turn up volume and turn off computer. Move to another computer, preferably a good ways down, and wait for someone to turn that computer on. Without a doubt everyone in that computer lab will be staring at that person thinking that they are publicly looking at dirty things, and they will be doing everything possible to turn that dang thing off.
P.S. You can make the sound clip anything you want. A clip of a loud yell or bang is really funny too. People are never expecting a really loud bang when they turn on a computer. The one i picked just happens to be the most awkward ensuing clip.