Do you have any other devices attached that are occupying a drive letter (slave drive, CD or DVD, etc)? It's possible that the external device is trying to asign to a drive letter already taken (had this happen to me in the past). If so, you may need to manually asign new drive letters to the conflicting devices, even if temporarily to check.
Refresh USB Ports Use Drive clean up to remove USB device drive info from the registry click here or In device manager Uninstall EVERYTHING under the heading USB, reboot the PC let XP find and install new hardware. Your equipment should now be recognised.