Have you tried deleting the drives and letting Windows re-discover them ?
Worked for me in a similar situation but I know its not always effective (that is
to say we are clutching at straws here !). It seems - as you have observed and so
have I - that drives can get into some sort of twilight zone where they are known
by XP to exist but are not fully integrated so they get whatever letter is next available
when they appear in the boot sequence.
