Rotating the screen

Is it possible to rotate the screen? I want to be able to switch between looking an object right side up and upside down. The same way you rotate a drawing or a painting around to look for errors.

Go to User Preferences (Ctrl+Alt+U) and under Input set Orbit Style to Trackball

I don’t mean rotating the 3D camera, I mean the window displaying the object. Like this: M -> W. Or like flipping a picture upside down in photoshop.

EDIT:

http://i1187.photobucket.com/albums/z394/knives4eva2486/Flip.jpg

I don’t mean rotating the 3D camera, I mean the window displaying the object

If by 3d camera you mean 3d view, then it’s the same as the window displaying the object.

Unfortunately there isn’t a 1 button solution for this.
With Numpad 2 & 8 you rotate the view up & down. By default this is in steps of 15degrees. But in the User Preferences you can set this to 90degrees (look for Rotation Angle under Interface). Then combine Numpad 2 & 8 with Numpad 4 & 6 to get the view you want

If you’re thinking of the “window” as a discrete object that can be manipulated in 2D space (like a painting within a physical frame), then no, the rotations you describe can’t be done. As Sago says, the window is where the 3D camera view is presented, and the camera view is what can be manipulated.