XP has like 40% of the OS market. So much so that the UK government paid M$ 8 million to extend support for XP.
There are some companies I contract for that has NT4 still in active operation. You have to look at the cost of upgrading of an extremely stable OS, in this economy. The ROI on infrastructure alone makes companies slow to upgrade or migrate.
I know that NT4 is extreme for this argument, but there are hundreds of custom applications that do not work in windows 7/8. Microsoft is rubbing its greedy hands together for the corporate support revenue.