MS-DOS is also an operating system. Earlier versions of Windows could arguably be called "shells", rather than actual operating systems. However, Windows 95, 98, and ME primarily use MS-DOS as just a...
Far from it. MS-Dos (The MS stands for Microsoft or M$soft) and is covered by probably a million patents. It started as QDOS - (Quick and dirty Operating System) and in most eyes has stayed that way.