Drive Letter for CD-ROM Won't Stay Changed Under Windows XP

I have CD-R/W and DVD-R/W drives which I long ago assigned as drives K:
and L: (by using Computer Management > Disk Management > Change Drive
Letter and Path); they have had these assignments for the past couple
of years. Recently I noticed that one drive was assigned as G:, rather
than L:. I changed it back to L:, but the next time I booted up it was
back to G:. I've now changed it multiple times, but the asignment to L:
(or any other letter I try) only lasts until I reboot, when it goes
back to G: I also tried fixing this problem by editing the registry
(HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices\DosDevices\G, but again it
reverted to G: after rebooting. I tired assigning the drive to L:, then
connecting a USB hard drive and assigning it G:, but after rebooting,
the CD was again back to G:, and the USB drive became H: (Note: My hard
drive has 4 partitions, C:, D:, E:, and F:, so G: is the first
unassigned letter.)