Without any hard evidence, I believe that XP is still the most used O/S in the world for the following reasons:-
1) Vista bummed, most IT professionals or large organisations do not like or use Vista. The only people that have Vista are those that have recently bought new PCs.
2) XP is the O/S of choice for most Netbook users.
3) Windows 7 has not been released yet but, in order to tempt corporate users, MS promises to incorporate an XP mode within the new O/S.
Windows 7's XP Mode: It's almost ready - Software - Breaking Business and Technology News at silicon.com
4) Third world countries prefer their PCs to have XP installed on them rather than any other O/S.
If anyone can counter my reasoning then please do.
UK Bob