Pierre Greborio points me to this great guideline from the MAC World: Introduction to the Apple Human Interface Guideline. Apple has an operating system, Mac OS X, which combines a powerful core foundation with a compelling user interface called Aqua.
Aqua defines the standard appearance of specific user interface components such as windows, menus, and controls, and is also characterized by the anti-aliased appearance of text and graphics, shadowing, transparency, and careful use of color.
This documents describes how to obtain an interface like this... maybe Longhorn will be so good in UI? (Aqua is a fast interface and requires not too much hardware resources... I hope the future Windows UI will have not too much requirements).