Also MS doesn't sell computers they sell software, they would have to go after Dell and the others to offer an alternative even if that alternative didn't sell very well. i.e. Linspire.
Actually, per the terms of the anti-trust settlements, they can't. They can't force any vendor who buys Windows to also offer an additional piece of software. They got busted on that before, too, and they won't be doing that again.
So, Microsoft *has* to make sure that Apple stays healthy. Why the heck do you think they even continue to make Office for Mac? It's not because they *want* to give Apple more inroads into the workplace, it's because if they don't they screw themselves.