Answer:
The letter used by Windows XP (ie a, b, c, d, e etc) is essentially a reference.
By default, letters 'a' and 'b' are floppy drives and drive 'c' is the hard drive that the operating systems (Windows XP in this case) is stored.
The other letters are used whenever Windows XP detects another drive. if you have a CD/DVD drive in your computer it is liekly to be displayed as drive 'd'.