I don't know about register but if you made enough hardware changes to an XP set-up you have to activate it again.
It's possible that you've run into Microsoft's new anti-piracy requirement for OEM builds where they check to see if you got one of the pirated numbers, which is how they know the serial numbers are being counterfeited.
About all those drives.
You may need to update the drivers for the controller card or replace it with another for it to run under XP.
Read this and see if it's any help :
http://support.microsoft.com/?scid=k...d=1173&sid=467