I think you need to brush up on your definition of 'monopoly'
Monopoly - In Economics, monopoly (also "Pure monopoly") is a persistent situation where there is only one provider of a product or service in a particular market. Monopolies are characterized by a lack of economic competition for the good or service that they provide and a lack of viable substitute goods. [1] Alternatively (a modern and less common usage), it may be used as a verb or adjective to refer to the process (see Monopolism) by which a firm gains persistently greater market share than what is expected under perfect competition. The latter usage of the term is invoked in the theory of monopolistic competition.
Anyway, as I stated, no one is forcing people to buy MS products, MS is a successful company that really grew large with the release of Windows 95. Till that time you had bunch of OSs around, you had OS/2, you had AmigaOS, you had Atari and CP/M, and you had Mac OS, MS wasn't that dominant back then, basically everyone had an equal chance, what made MS Windows 95 so successful was because it was an open architecture, it wasn;t tied up to proprietary parts like most of it's competition back then. Microsoft earned it, spent their time and money, and developed Win 95. I am not saying that MS is innocent and doesn't abuse it's power but to get punished simply for not releasing it's own code is a bit ridiculous to me. Bigger problem is MS buying out small innovative companies.
And as far as Mac OS X being open source, it's not as open as it used to be, Jobs made sure of that. I really don't see a big deal of an OS being open source or not. If you want to develop your own OS and give it away for free then it's your choice. I know if I was in charge of MS I wouldn't give anything away for free. You want to use/access certain part of a code you need to buy a license from me. As I said earlier, people are not limited to MS Windows/Vista, there are other choices, in this case I see a company being punished for being too big and too successful.