Windows XP does not take kindly to being moved from one PC to another.
Really, Windows is going to have a field day with all the hardware changes. Not to mention, it'll probably require re-activation once it boots, as the hardware changes will be so many, that it will think and rightly so its in a new PC.
I would suggest, you boot from the Windows Setup disc, and do a repair install. it will keep your files untouched, but you'll have to download all upgrades again and probably fix some of the software installations you had, and even then, its going to ask you for drivers for all the new hardware.
That is if it doesn't ask you for reactivation first, in which case you'll need to call Microsoft to get it reactivated.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.