The most widespread family of operating systems on the planet, produced by Microsoft Corporation.
This is more of a general platform than a single operating system - the differences between Windows 3.x and Windows XP are dramatic.
This Wiki has a fair amount of information on the subject of Microsoft Windows and Tcl and Microsoft Windows and Tk.
Interfacing to Windows/CE, recently renamed PocketPC, is something of a special case - but great fun! RS
"Despite being closed source, holes are discovered constantly. " [L1 ]