The first version of Windows released.
During development, the OS was known as "interface manager", and it's intended purpose was to be a graphical interface of MS-DOS, Production lasted for 2 years, and it was renamed "Windows" in 1983, it later got released in 1985.
The Operating System does have some features, that do stick with Windows in the future, such as Notepad, Calender, Clock, Control Panel, Paint and Write (known as Wordpad in later versions). The main explorer is the MS-DOS Executive, where you can access your main programs, and you can do Side-by-side windowing (similar to snap from Windows Vista and later). It was planned to have drag and drop (much like the Macintosh at the time), but due to copyright issues with Apple, it had to be changed for Windows 1.01.