This ones a cool one to play pranks on your friends, especially if your friends do not know how to get it back to the normal state 😉
The shortcut is
for normal windows : Ctrl + Alt + <Up arrow>
and for 180 deg it is : Ctrl + Alt + <Down arrow>
How ever you should know that these are default shortcuts for Intel Extreme Graphics Controller, and you can change them. To check / change or disable these keys go to control panel–>Intel Extreme Graphics–>Hot Keys.
Have fun :).
Some users have written to me saying the shortcuts do not work. This happens when you have not enabled rotation feature in Display Settings under Intel Graphics Media Accelerator media. You need to enable rotation in order to be able to Rotate / Flip your windows screen upside down / 90 / 180 / 270 degrees.