It started with being awarded producing the operating system for the IBM PC in the early 1980s. At that time everything labeled "IBM" had a big headstart among companies buying computers. As the IBM hardware was open, many clones could be built, all running MS-DOS. So MS could get away with their crappy OS even when the Mac was around, and cheaper alternatives to the Mac like Atari ST or Amiga became available. When MS came out with Windows 3 in 1990, the suits were confirmed in their decision to buy MS-powered stuff, as Windows was able to cover Mac functionality in a way. Also they developed Word in the 1980s, and added other Office components (mostly by buying companies) resulting in a really usable suite, they just had to win.