Microsoft's monopoly power originated from their success with Windows, and was used to abuse competitors later on. Without the initial success of Windows, they wouldn't have had that power. Make no mistakes, Microsoft was evil (and probably would be if it wasn't such a shell of its former self), but the initial success of Windows was (more or less) legitimately won.
Remember DOS? The operating system, if you could call it that, of the original IBM PC, and MS-DOS, the OS that ran on all the IBM-compatible clones? The PCs that took the market away from Apple? The ones that established Microsoft's monopoly?
The market was different back then. There were a lot of choices, like PC-DOS, and even other computer systems like Mac, Atari, Amiga etc. MS-DOS itself did not give them a monopoly. It was an advantage of course, but not a "competition killer". It could have been IBM's OS/2 that would have "won", or a world with more cross platform development.
