I think the problem with XP, regardless of the version you have, is all the updates that have been created for it. From my own version, and the one I use at work, I find it runs incredibly slowly compared to Vista or Win 7 for that matter. It will come to a head that companies will have no choice but to upgrade to a newer OS.
Also, I hear on the grapevine, that Microsoft are withdrawing free support for XP as well, which I believe is a calculated move to get people upgrading pretty sharpish. If they produced something that didn't have so many flaws I wouldn't have a problem with this. Vista was a PR disaster, though now running very well, Win 7..... well, time will tell I guess.