Even if you can change the drive letter of the XP drive, which you cannot, you will make it useless since there are hard-coded links to the content on the XP drive. If you change the drive letter, all those links will be to a non-existing, or incorrect, drive resulting in malfunction.
Your best bet is to disconnect the other drives in your system and re-install XP. It is always best to disconnect the other drives when you install just for this reason.