kryptonite;
You want to replace Windows MCE with XP Pro? Or are you trying to replace Vista Ultimate with XP Pro?
Vista Ultimate is essentially the new Windows MCE (though more stable and reliable). The three versions of XP (Home, Pro and MCE) were essentially stepped versions... Pro had all of the same architechture of home with added security, utilities and networking capabilities; MCE was (supposed to be) the same as Pro with added media portal and multimedia management. In essesnce; MCE had all of the features that a Pro user would need with the ability to enjoy multimedia experience. I personally have never used MCE, however other users of it I know have had mixed experiences... some folks like it, others tell me it's an inferior OS. XP x64 was an entirely different animal... Microsoft's 'first taste' of 64bit with an XP user interface *different enough to require it's own drivers for third party hardware and software because the regular XP driver's usually won't work*.
For you Vista users who are scrambling around looking for drivers for your peripherals, you may want to try loading any x64 drivers that the third party hardware vendors might have made available. Often times those x64 drivers may give you at least some functionality of your hardware until they are able to release more reliable Vista drivers.
If you are infact trying to replace MCE with Pro (if you are a power-user, then I can't say that I blame you) then the procedure should be fairly straight-forward, though you may need to f6 your chipset drivers at the beginning of setup so that XP will be able to find your hard drive. If this doesn't work, then you'll probably need to get hold of a full SP2 version of XP.