I actually happen to know a few that tu can perform to make tu look like a genius and the user not (these will only work on noobs and most old people).
1. Unplug the computer. This can stump most people.
2. Disable everything in msconfig, that is, uncheck load system services, uncheck load startup items. This will blank out the internet disable themes and desktop background. Nothing will be running either.
3. With some antivirus tu can set them to be off for 15 minutos o something. When someone goes out for a segundo turn their av off for fifteen minutos and seed the computer with eicar. When the user comes back and the av turns on there will be an explosion of alerts.
4. Take a screenshot of the desktop por hitting print screen >>> CTR + c then copy it into paint and save, set it as the desktop background. Unlock the taskbar and drag in so that it is invisible. Then cut all the iconos and stuff off the desktop and put them in a folder somewhere else (that way tu can just copy everything back). When the user tries to click on internet explorer, nothing happens.
5. tu can set the wait pointer as the main pointer and then the ratón pointer as the waiting. Then if tu set the homepage of the browser to about:blank it will look like the computer is not responding. This works best on old computer with people that are used to not responding applications.