>> "I own two machines which cannot be upgraded for very good reasons."
> What are those?
Plenty of reasons. Khyber's comment below about hardware drivers is one. If you have a sweet server that's still chugging along, you feel no need to replace or upgrade it. If you did, though, you'd have a time finding drivers for it.
Another reason is if you're using a very expensive software package that simply won't work with anything newer than Windows XP. Then it's not just a simple matter of upgrading Windows, but having to shell out tons of money for other software upgrades at the same time. Until the economy turns around, that ain't gonna happen.
We've run across cases where a software vendor will say, "don't install anything newer than service pack 2." We handle it by completely isolating these machines from the Internet and disallowing the use of external, user-supplied storage (which most smart admins do anyway, on general principle).
Here's a piece of trivia for you: one of the key audio streaming companies* for broadcast radio stations, as late as last year, made it clear in their contract that they would ONLY support Windows XP. We dropped them for that reason, but folks, this was in 2012. That kind of stuff still happens, too, and again, blame the economy.
This admittedly won't affect most users, but it does affect some of us.
(*actually, to be technically correct, they're an ad-insertion company -- they insert commercial inventory in your online stream -- but I figured everyone's eyes would glaze over if I tried to get that detailed.) :)