Found this interesting site, called Guidebook.
Welcome to guidebook, a website dedicated to preserving and showcasing Graphical User Interfaces.
Lots of nostalgic screen-shots & descriptions – of old Windows 3.1, OS/2, NT 3.51 – and even up to today & beyond – with Longhorn, etc.
Amazing to see how things change & evolve – and that October will be a 20th anniversary for Microsoft Windows.
The timeline is the most useful part – can then jump to the various OS screen.