1) Dell sells the OEM version of windows. This is not the same as you would buy if you walked into a computer store. It's very likely that microsoft gives them huge discounts because:
- Dell are more likely to stick with just MS (it's an incentive), thus maintaining MS's dominance (and it's easy for dell).
- Even if they only sell it to dell for $20, they sell many thousands, so they still make lots of money.

2) who knows? MS probably exerts some influence to convince dell NOT to sell computers with anything but windows. This makes sense for MS for commercial reasons. They can pressure dell with this. Windows is very popular. people just expect to have it now and the cost is factored into the price of hte PC.

It's also very cheap (for dell).

And it's uniform. It means dell has only to provide support for a single family of OSes. This makes tech support etc easier and hence cheaper.

It's all about the money! :-)

cheers