Hello,
You're talking about Windows Flip 3D (where a 3D representation of windows is shown in the screen for task switching).
Ensuring that you have Windows Aero visual style / user interface enabled, you press Windows Key + Tab to task switch using Windows Flip 3D.
Pressing Alt + Tab brings up Windows Flip task switching (the traditional 2D left to right representation of running programs), not Windows Flip 3D.