In the 1990s, Microsoft had two largely independent OSes, both branded as "Windows":
- The original "Microsoft Windows", which was a graphical shell on top of MS-DOS
- A completely new operating system, which originated in a collaboration with IBM on OS/2, and was internally code named "NT", based on the hardware they were testing with.
The first version of the new OS was marketed as "Windows NT 3.1", to imply that it was similar to the existing "Windows 3.1", even though it was essentially a different product - they could just as easily have called it "Microsoft NTOS 1.0". The main thing the two "Windows" products shared was the "look and feel", although there was some support for running the same programs on both OSes.
The two products then continued in parallel for a while: the DOS-based OS (with large parts of MS-DOS itself replaced) had releases named "Windows 95", "Windows 98", and "Windows Me"; while the NT-based OS had releases named "Windows NT 4.0" and "Windows 2000". The two products continued to share a "look and feel", and a few components, but still had fundamentally different internal architectures.
At this point, Microsoft abandoned the DOS-based product, and continued only the NT-based one. This is the OS that is still branded as "Windows" today: "Windows XP", "Windows Vista", "Windows 7", "Windows 8", "Windows 10", and "Windows 11" are all continuations of that project.
Since they've never "thrown it away and started again" since the original "Windows NT 3.1", those OSes are still internally labelled "Windows NT" in various places.