GUIdebook
If you're in the slightest bit interested in GUI design, this website is definitely worth bookmarking. It has a historical overview of Windows 1.0 interface right through to Longhorn. It also covers the graphical evolution of other operating systems such as the MacOS, OpenStep, Rhapsody and many others.
Here is the link: http://www.aci.com.pl/mwichary/guidebook/index