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??