|Is DOS an operating system ... ?|
The clue's in the name! As Hzj jie said: DOS = Disk Operating System
|MS-DOS was a modified version of DOS that was only designed to start Windows|
Microsoft DOS wasn't invented to launch Windows. It was "developed" as the O/S for IBM's new personal computer back in 1981. The idea of building Windows on top of it (using its file system services) only happened later (with Windows 1.0 in 1995.)
(I used "developed" (in quotes) as I knew Microsoft bought a more or less finished version of DOS from another company, but didn't recall who it was. Thanks to Wikipedia.org, I now know it was Seattle Computer Products. Microsoft bought 86-DOS (aka Q-DOS) and renamed it to MS DOS; so "MS DOS" did predate Microsoft! To confuse matters, IBM then sold it on as PC-DOS.)
|It was used to launch Windows, but could run on its own, which Windows could not do until about the year 2000, |
16-bit versions of Windows (up to Windows 3.1) ran on top of DOS. That is, they relied on DOS for file and disk access. You needed a DOS installation for old 16-bit versions of Windows to run.
With Windows 95, etc, DOS was relegated to just handling 16-bit drivers; Windows took over file system handling, and handled 32-bit device drivers itself (VxDs). And you didn't have to install DOS separately; it was provided as part of the overall Windows installation, allowing you to boot in command line "DOS" mode.
But Windows NT
3.1 was released in 1993 -- and it never relied on DOS. So some versions of Windows have been running DOS-free since the early 1990's.
When you run a 16-bit program on Windows NT or the 32-bit versions of modern Windows, it's being run by WOW (the Windows on Windows) compatibility layer. (64-bit versions of Windows don't support WOW; just WOW64, which allows 32-bit apps to run on 64-bit Windows.)
(With Windows 98 and Windows 2000, the Windows Driver Model took over from VxDs and the Windows NT driver model. WDM is actually an enhanced version of the NT model.)
PS The current version of Windows evolved from Windows NT not Windows 95. Some the code was common, mostly in the user layer, but the kernels of Windows NT and Windows 95 were radically different. And it's the Windows NT kernel which was taken forwards.