In 1980's and 1990's DOS OS was prominent. By the end of 1990's Microsoft developed Visual Basic Language which is the basic language for developing windows apps. So, the problem at that time was "Most of the computer users are dependent on MS DOS, but they want to use and develop windows apps in parallel". Keeping this in mind Microsoft allowed the DOS users to run the windows apps on DOS OS. But to allow this Dual platform ( only DOS and Windows) support Microsoft developed Windows Runtime (Core for Windows OS to run of any version) . When the DOS user installs windows runtime, then he can work with windows apps in parallel. So we can say windows sit on top of DOS.

      Now at the start of this millennium (mostly 2001) Microsoft developed the .NET Runtime which has similar functionality to that of Windows runtime but this is Cross platform (Windows or any other OS)supporter and allows programmers portability (a C++ programmer can easily develop a .NET App). So we can say that .NET runtime sits on top of Windows (or any other OS).

    Note: To work with Windows apps Microsoft designed VisualStudio.NET which is needed to be installed on DOS OS to run windows apps and now in Windows OS to run Windows, Web etc. apps.

    So, the credit for initiating the concepts of cross platform support and developing windows goes only to Microsoft. Microsoft Hip Hip Hurray!!!