Microsoft banned the sale of XP with new machines at the end of last June. They can still be bought with a downgrade license for business use (as alan14 says), but not for personal. You will probably need to show it is to be used for business purposes, that is have a business account with the likes of Dell. But, must ask the obvious question, why the heck do you want XP? It's history now, and Vista leaves it standing in all areas.
Ihave tried Vista Home & Ultimate not a problem but 64 bit ultimate needs a lot of questions asking to varies software suppliers Compatability? All the Vista systems work OK for home & gaming use XP will be totally lost next year maybe the credit crunch will keep it going this year