How do you turn the image on a computer screen upside down?

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One of the girls at work just scored a pretty good "gotcha" against me. I stepped out of the office for a minute, and once I got back, I found that the display on my PC's monitor was now upside down! The girl who did this came back, hit two keys on my keyboard (I didn't see which two) and the display righted itself. Does anyone know how this would have been done? And has anyone got any good ideas for a revenge "gotcha"?