I'd say a majority of users never install an OS. They get what comes on their computer and stick with it until they buy a new computer.
This is exactly right. Also - PC, why are you talking about re-installing OSes? Those numbers aren't affected by reinstalls, they're measured (at least i'm pretty sure) by a combination of sales and web use. It may actually be JUST web use.
But yes, XP share is largely corporate machines and old personal machines as the vast majority of users only "upgrade" their OS in the scenario Nathan mentioned. Where I work, we just started supporting Windows 7 on new machine purchases last month.