Everyone knows Microsoft is going to end support for Windows XP on April 8, so most XP users are upgrading to Windows 7 and some are going to fot latest one (Windows 8). But lots of users are not willing to upgrade.
After Windows 7, Windows XP is most popular Operating System. (Maybe because of this reason Microsoft is moving to Windows 9 instead of further development of Windows 8.*)
Have a look on picture below (by avast!) which explains current usage of Windows XP.