Windows will automatically assign the drive letter when you've created a partition on the new drive (Or you can choose a custom one that's not in use (except C))
Anyway, installation should go something along these lines:
Plug it in.
When booted into Windows, right click My Computer then choose Manage
Go to Disk Management and scroll down to your hard drive, right click that and initialise, go through the wizard to format and partition
Job done