It will just take the next available alphabetical letter. (although I think that Windows defaults to C for the primary hard drive and misses B out unless theres 2 floppys.
eg
A = Floppy
B = Floppy
C = Primary HDD
D = CD 1
E = CD 2
F = CD 3
G = CD4
H = Slave HDD
Although on installation I think that you can specify the letter but I cant quite remember.
I've always wondered what windows would do if it ran out of letters for drives??
"No pressure, no diamonds" Thomas Carlyle