If you have a powerful computer and Windows Vista Aero (that 3D, glass-like window interface) is enabled, you can flip through your windows in 3D. Instead of using Alt-Tab to flip between windows, hold down the Windows key (it’s near the Alt key) and press Tab repeatedly. Windows Vista will show your windows in 3D, even if they’re changing.
If it’s not working for you, first make sure that your pressing Windows+Tab, and not Alt+Tab.
If you’re pressing the right keys, Aero might not be enabled (3D-Flip requires Aero). Follow these steps to troubleshoot Aero.