Enable desktop composition. The first is responsible for the translucent "peek" behavior and the second for the ability for alt+tab to show what DWC (desktop window composition) is getting as the image to show for the program in question. Without desktop composition, you won't get the preview icons.
Alt+Tab ↹ is the common name for a keyboard shortcut that has been in Microsoft Windows since Windows 3.0. This shortcut switches between top-level windows without using the mouse; hence it was named Task Switcher (Flip in Windows Vista).