Some of you might still want to stick with windows xp instead of windows vista but unluckily nowadays, there are almost no computer manufacturer sell their product with windows xp in it.
If you’re about to buy a new computer with vista in it, make sure you got windows vista business or windows vista ultimate.
You can downgrade your vista business and ultimate license to windows xp pro without adding a single cent.
OEM downgrade rights for desktop PC operating systems apply to Windows Vista Business and Windows Vista Ultimate as stated in the License Terms. Please note, OEM downgrade versions of Windows Vista Business and Windows Vista Ultimate are limited to Windows XP Professional (including Windows XP Tablet PC Edition and Windows XP x64 Edition). End users can use the following media for their downgrade: Volume Licensing media (provided the end user has a Volume Licensing agreement), retail (FPP), or system builder hologram CD (provided the software is acquired in accordance with the Microsoft OEM System Builder License). Use of the downgraded operating system is governed by the Windows Vista Business License Terms, and the end user cannot use both the downgrade operating system and Windows Vista Business. There are no downgrade rights granted for Windows Vista Home Basic or Windows Vista Home Premium.
For even more detailed information (pdf file) look HERE