It's hard to understand why people are still using vista and xp
I can see the winky but, apart from the thread will get a little derailed, why would you think that...?
The only reason that my father in law changed from XP to Windows 7 is that it came pre installed on his laptop. Unfortunately Windows XP will not run on it and this has forced him to get rid of his fully working Genus scanner and separate printer as there are no 64bit drivers for those devices.
Then can be many "compelling" reasons for some people to change to Windows 7 but there are others who would question the need to change just for change sake with no perceivable advantage.
In the context of an HTPC I find that XBMC runs perfectly well when on Linux, XP, Vista, Windows 7 or even the Windows 8 preview. In that situation surely the OS, from the end users perspective, is merely a conduit to facilitate the watching of TV and thus the age of the OS does not matter.?