In every revision of MS-DOS--and later, Windows--the OS has become more complex. Each major revision of Windows requires an army to revise and test the OS and to write new drivers for devices that connect to the OS. This has created enormous barriers for new firms wanting to enter the OS market.

In the past, Microsoft used its control over the PC OS to eliminate or dominate a number of innovative rivals in markets for important desktop applications. By constantly modifying the underlying OS, Microsoft alone 1) is in a position to fully utilize its functionality, and 2) has the opportunity and the motive to design revisions of the OS so that rival products perform poorly or even crash. With the constantly evolving nature of the OS, Microsoft's competitors are always standing on shifting sand.

--James Love