Actually they could sell laptops that are optimized to really work with their OS - they would have more control of components and drivers. They could also sell Macs with Windows loaded on them as a re-seller.
I think that the anti-trust people would have a lot to say to Microsoft if they did that... not to mention the lawsuits from HP, Dell, ASUS, Compaq, and every other computer maker that would find themselves competing in the hardware field against their own OS supplier.