Aero was introduced by Microsoft in Windows Vista as a new graphical user interface with eye candy design and effects, like the ability to change between minimized applications by rolling their windows in 3D.
Now you can have that rolling effect when alt-tabbing between apps without having to upgrade your dear XP to Vista. Just install this simple tool and you'll get the same effect every time you press alt-tab or click on the program's icon.
Plus, you can choose between the Vista or the Mac layouts.
Although Shock Aero is a good alternative for all those non-Vista users, the 3D effect it tries to emulate is still far from the real one.