Microsoft developed DOS from CP/M, Windows 2-3 (WIN16), Windows 95-98-XP-Vista-7-8 (WIN32), Windows NT (on x86, Solaris, DEC Alpha), PocketPC / HandheldPC (x86, MIPS, SH-3, ARM architectures) and developed applications on their own operating systems and other peoples. They are, without argument, the single most experienced operating system company in the world.
Not quite! They bought a BASIC language which they converted into the original DOS. CP/M was the dominant OS of the day, next to Unix anyway. It's the OS I learned to program under 380Z running CP/M.
Windows 95-98, ME, XP, VISTA, 7-8 - these were mostly MS's creations. The 2 direst of them ME and Vista were examples of what happened when MS tried to do things on its own without buying the tech to build them on first.
NT, 2000, Server - these were built on NT which they bought from a company called NT.