Did you say Windows XP?
Why is this important to you?
This is documented in the Microsoft KB, but it only applies to a case where you really need to change then drive letter. That is often when you have more hat one instance of the same OS on one drive and the second instance was made by a disk clone thing. Inn other words, it wants to be the C: drive.
It is done by edits to the registry. Are you willing to edit the registry? Unless there is a real problem, better to just leave it alone.