A better idea for microsoft would be to get back into writing great software. Their main business as always been software. If they can do this they don't have to buy anyone. Buying these companies doesn't mean they'll get all talent with them.
It'd probably be a good first move to get rid of half the staff - should speed things up a bit.
However, writing an OS I don't think fast is necessarily good. The biggest improvement (IMO) microsoft could make to windows would essentially be a completely error free XP. I've tried Vista and I've seen no appreciable improvement and just a huge pain in the ass for doing absolutely everything I'm used to.
I think if they could make XP survive anything that can be thrown at it and virus proof the sucker to hell, I think it would be the best product MS ever made.