A lot has changed from the early days of MS-DOS all the way up to today’s Windows XP. One thing that hasn’t changed is the disk lettering system. That’s kind of weird!
A and B are reserved for floppies, C is the primary hard drive, and D through Z are for everything else.
It’s as clear a way as anything that I can think of to label drives, but it’s funny that it hasn’t changed in so long.