Before Microsoft, the plethora of operating systems made it well-nigh impossible to move programs from one make of computer to another. As a result software was from 10 to 100 times more expensive.
Whether or not Microsoft has stepped over the mark, it deserves some credit for having done more good than harm. Today no one asks if the program they are buying will work on their make of computer. This could not have been achieved without some degree of monopoly.