50-60 % that for whatever reason are still on XP
One of those reasons is sheer pig-headed stubborness: "XP works fine for me, so I ain't upgrading."
Another reason is learning new stuff and a fear of the unknown: like Vista/Win 7 is gonna fuckin' bite 'em, or something.
Also, hearing it from a friend who, heard it from a friend, who heard it from a friend that their bosses daughter heard it from her 4th cousin that somebody told him Vista/Win 7 was no darned good.
This is partly why development has been slow or non-existent for subsequent Windows OSes, particularly with smaller 3rd party devs. With so many having an icy grip on XP, they see no market to develop for... and those who have moved to more capable tech with greater potential, well they pay the price for this narrow-minded pig-headedness.
Granted, some people genuinely can afford to upgrade, but then I've heard the whiny ass complaint "but I can't afford it" coming from some who spend way more a year on take-aways, beer and pizza... far more than the cost of an OS upgrade. It's these people who hold OS/softeare development back, and if I were MS and/or one of its partners, I'd have pulled the plug on XP 2 - 3 years ago. Yeah, let people keep/run XP if they want to remain in the Dark Ages, that's to their own disadvantage, but focus all development and resourses on Win 7 and the future.