Here's an app that will fix that for you...
USB drive letter manager - USBDLM
See the details:
"For a drive which XP has no drive letter assignment stored it assigns the first availlable drive letter. The search goes upwards in the alphabet and starts at 'A' for floppy drives, at 'D' for CD-ROM drives and at 'C' for all other types.
XP can save exactely one assignment per drive letter. So when a drive is attached again it gets the same letter as before. But the drive letter is not reserved while a drive is not present and therefore reassigned to another drive if required. When this happens the former assingment is overwritten and the first drive get the first availlable drive letter again even its former drive letter is availlable at the it's beeing attached. The assingnment just doesn't exist anymore.
Net and substs drives are completely unconsidered by XP here. XP has no problem assingning a drive to letter which is currently used as network share!" ref:
Tips for USB pen drives