In 1999, Microsoft made a Windows version that would make the company skyrocket in money. This was actually a series of NT-based Windows OSes, and they were open-source. It is known as NOT (New Open Technology), or just OpenNT. These NOT-based operating systems come in templates, to make your own Windows version, and also an application to customize the disc file.
In 1997, OpenNT began development. Tho most of the builds were internal, some were leaked and shared. Some builds would get publicly released by Microsoft.
To get the template and the application for customization, you need to do a form to be in the Windows Open Source Program. You need to say why would you be in this program, how can you revolutionize Windows, and you need to have experience in prior Windows modding. If you fufill all those requirements, you will be in the Windows Open Source Program. Some lucky members can be chosen to be a beta tester for the Windows Beta Open Source Program, which is the same thing, but Microsoft releases weekly betas to test and report bugs.
In 2008, the OpenNT project was cancelled, due to lack of interests.