I find it best to delete the old partition.
So you delete the partition (D is the command, I think), then I believe you press L to confirm that.
Then create a new partition (C) in the unpartitioned space (which, you just deleted), and choose to setup Windows on that partition, formatting it with NTFS.
Those are the steps, I'm pretty sure the commands (D an L) are right. Either way it is pretty intuitive as you go through; just remember to Delete, Create, Setup, Format.