I don't think it's a helpful comparison to make. The only similarity between the two is the market share - android being open source changes quite a lot.
Not that much. Most people don't install an OS on their phone other than the one the carrier puts on it. The big differentiator for Windows was that it was an open ecosystem. Anyone could make an app. Anyone could install any app they wanted. Anyone could make a PC, even individuals. And Windows was just the glue holding the ecosystem together. Android seems to be slotting into a similar role for smartphones today.
