Depends how you do it. If, in Windows setup, you delete the partition
and immediately recreate it, it remains as D. If, on the other hand,
it was already deleted when you run Windows setup, then when you
create it, it will become F (the CD drive will be E). There's no
reason to delete it; just format it during Windows setup.
Pay attention to the setup screen that shows the disks and their
partitions. Each partition will have a drive letter assigned to it.
Those are the drive letters that Windows XP will have after it has
been installed.