All drive letters can easily be changed from within XP, ecept that which holds the system files (usually C). There is a way even to do this though. To change any other drive right click My Computer and choose Manage, then Disc Managment. Right click and drive in the bottom half of the window and choose to change drive letter.
To do it for the system drive click here Not for the faint-hearted though. You really need to find out why the default letter for the system files is not "C"