Well, corporate users is a big part of that. We just had our systems upgraded to XP from 2000 a few months ago.
Also, many people don't reinstall their OS. And frankly, I don't think that it should be a requirement to reinstall an OS every year or whenever.
Many people still don't want to upgrade to 7 because either the backlash from hearing how "bad" Vista was, they don't want to buy an OS upgrade, and their system won't handle it. 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.
I've got 2 people at work who just bought new computers because their old ones just got so slow, even after they went to a website that popped up that would fix it (
Now they want to wipe it before throwing it out. I'm thinking of offering to do it for them if they let me keep the old machines, once I find out what they are. I know one is from 2002 so it will be basically useless, but I don't know how old the other one is.