I think MS has a greater grip on the OS market than a normal monopoly. and here's why:

Assume Windows is installed on most computers.
->Programs written for windows do best,
-->so most programs are written for windows,
-->so you need windows to run most programs,
-->and windows continues to be installed on most computers.

->Additionally, most people learn to use windows,
-->so they avoid alternatives because it's too much new to learn,
-->and windows continues to be installed on most computers.

Now a windows compatible "translation layer" (wine) is being written but that truly takes a fucking long time, and heaps of work.