It is the Mitsumi drive, as N7 said. I've got one in the system I just put together. For those who don't know, this is a combo floppy drive and 7-in-1 memory card reader. It transfers the floppy data via standard floppy connector, and the memory card data via USB. Windows sees the two memory card slots as USB storage devices and assigns them drive letters. On your system, Windows must have seen the Mitsumi before the hard drive and assigned them letters first. My system has the Mitsumi, the same DVD burner you have, a Samsung SM352B CD combo drive, and two SATA hard drives. I'm only just test-installing XP at the moment just to see if everything works, but I noticed that it assigns the Mitsumi card slots letters before the IDE optical drives, but after the SATA hard drives. If your hard drive is PATA, that would make sense and be the explanation.
Since I'm just about to re-install XP anyway and start installing all my old apps, I'll try to see if I can chance the drive letter of the system partition first (which is C for me). I can't mimic your exact condition, but I can see if I can change the drive letter.
Here's another idea for you though! Disconnect the USB cable from the Mitsumi to the motherboard before booting, or better yet, disable the USB in BIOS so you don't have to physically unplug anything. My guess is that the drive will get assigned "C". You might need to disable the optical drive as well, depending on whether it is getting assigned a letter before or after the hard drive. However you do it, if the hard drive is the only visible device, it might very well come up as "C". Then you could install the driver, reboot once in case the driver install needs to do something on reboot, then shut down, reenable whatever devices you had to disable, power up, and you should be ok (I hope).
If that works, that'll be the first time I've been able to answer something on here!

Then someone can tell me why Cool 'N Quiet isn't working on my system...
-Joe