I hope the OP doesn't mind a friendly side debate.
Corporations don't like to update their systems is another reason.
I can certainly appreciate that. But I don't think that's as big a reason. If it were, then for those corporations that support mixed OS versions, I'd expect that many machines purchased at the time when Vista was pre-installed would still be using Vista. For those that prefer to be homogenous, many that rolled out Vista would also still be using it. That should result in a smoother distribution, rather than the shrinkingly small Vista usage compared to versions on either side of it. Vista machines, in particular, were upgraded as soon as possible - even preferentially over XP. Which suggests that quality of an OS is more important.
Also, I have an application used by a few dozen corporations. That may be a relatively small sample set, but as far as I know they dropped XP years ago. They're all running Win7 now. Working in corporate IT, you get flak if you upgrade too often, but you get more
if you upgrade too infrequently. At this point I just can't imagine so many XP holdouts existing in the corporate world.
But I could be wrong.