I can't believe w're still using Windows XP at my office to be honest. Operating in 37 countries with 35,000 employees you reckon they'd have something a little more "cutting edge". We're still using.. get this.. Lotus Notes!! 
You've given the reason right here ^
With that many people, upgrading PCs and licences costs a huge amount. Also when operating in other countries you also have to factor in language support. Some languages aren't supported very well and some fonts don't work at all with certain software.
My work are only just moving to Windows 7 but it's being done as part of an office move so all the old XP computers are being left in the old building and the new building is having completely new PCs with Windows 7 and this has been factored into the moving cost. We're also changing over a number of our major software packages from stuff that works on XP to stuff than works on Windows 7.
Other parts of our organisation will only get the upgrade to Win7 when they do small scale PC upgrades.
We "upgraded" to Outlook 2010 (from an older version) earlier in the year and it's cause no end of problems and struggles to run on some of our Windows XP computers, but on Win7 it's fine.
As for Windows 8, no business would touch it till it's been proved to be stable and secure and all your software runs on it properly.
