Flip 3D is part of the Windows Aero experience. If you find your computer does not support Aero, or if you are using a color scheme other than Windows Aero, you can view the open programs and windows on your computer by pressing ALT+TAB. To move through the open windows quickly, you can press the TAB key, press the arrow keys, or use your mouse to get the job done.
Windows Vista had a graphical feature called Flip3D for switching between open applications. The feature was located on the Taskbar. However, for some reason that feature is no more to be seen on the Windows 7 Taskbar. The characteristic of Windows Flip 3D is that, you can quickly preview all of your open windows (for example, open files, folders, and documents) without having to click the taskbar. Flip 3D displays your open windows in a stack. At the top of the stack, you’ll see one open window. To see other windows, you can flip through the stack.
If, for some unknown reason Flip 3D has stopped working on your computer, you can try the following.
Fix: Flip 3D is not working in Windows
If Flip3D has stopped working, then copy-paste the following command in Vista’s Start Menu Search bar, and hit Enter.
This will re-enable Aero by flushing it.
More at How to enable disabled Aero.