Microsoft Windows

Windows was previously a desktop environment for DOS and now it is an operating system using the Windows NT kernel.

Windows for DOS
During this time, Windows was a desktop environment and display server for DOS, an OS inspired by CP/M. It did include it's own graphics drivers and program API, however it used the MS-DOS API extensively and cannot be considered an operating system.

Windows 9x
In the 1990s, Microsoft developed several versions of Windows where the Microsoft DOS kernel was included and many DOS apps were replaced with Windows ones. Windows was an operating system that used the DOS kernel, essentially a distribution of Microsoft DOS.

Windows NT
Windows NT was a new kernel that also had operating systems named after it. Operating systems such as Windows NT 3.51 were the first to use the Win32 API later used in Windows 9x. Eventually Windows NT replaced Windows 9x as Microsoft's primary OS product, with Windows XP. All recent versions of Windows have used Windows NT as their kernel and API.