Assuming you have Vista Premium or above i think you meen a 3D Desktop??
If this is the case you need to enable Windows Aero first of all by right clicking the Desktp and selecting Personalize then it's in there somewhere.
To Flip through the various applications you are then running you toggle the Start and Tab keys on your keyboard at the same time then keeping the Start key pressed and selecting the required application using the Tab key.

Hope this helps.

Please note, not all graphics cards support Windows Aero.