Here’s another oldie, but I still get calls on this regularly.
I happens predominantly with computers using windows XP and Intel or NVidia graphics adaptors.

To fix the problem, press CTRL-ALT-UP.
To turn it upside down again, press CTRL-ALT-DOWN. To turn things to the left, CTRL-ALT-LEFT, and to turn it right, CTRL-ALT-RIGHT.

One gotcha. If you had rebooted to PC to try and fix the problem, you might need to log in first, before the keys will work.

FYI. There are 5.3 million hits on this google seach – http://www.google.com/search?hl=en&safe=off&rls=com.microsoft%3Aen-us&q=screen+upside+down&btnG=Search