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This will make the untrained computer user think there's something wrong with their computer. It only works on windows. it will make the computer almost unuseable untill you turn it back on. I suggest making them get you a soda first.
Step 1Task manager
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They must first leave their computer or you can ask if you can use it really fast. Once you have it click Control+Alt+Delete. this will get you to the task manager. On vista you must click after this "start task manager."
Wanna really screw with someone? Take a screenshot (PrtScr) of their desktop with the tasbar visible and everything. Now right click on the background and hide the desktop icons. Now drag the taskbar to the top of the screen and ato hide it. Now change the background picture to be the screenshot you took. Now it looks like they have their icons and taskbar, but they won't be able to get a response from anything there, because it is in fact just a picture. Just about as quick, and won't be fixed by a forced reset. It's a good time.
Wanna be reeeeally evil? Do all of the above, but make sure there is a porn window open when you take the screenshot.