Welcome to SF! BarrieNoodles
That's correct! Once you boot into a second Windows installation in a dual boot or even on a separate stand alone install to a different drive that will see itself as C.
For most having their optical drive remain as D regardless of which installation they are in is easy to take care of with a trip into the Disk Management tool and right clicking on a particular volume to select the change drive letter option. For seeing the optical drive made D while in use by another drive you would first change the other one from D to another letter like E, F, G, H, or another free letter available.
Once the D letter is free(if already taken) you can set that for the optical drive while a disk is present in the drive. For the second drive whiel in a dual boot most will set the E, F, or G drive letter for the host OS when booted in the second and the same for the second while booted in the host install. An external hard drive or usb flash drive when plugged in will then see E(external hard drives mostly) or H, I grabbed by flash drives
To see any of that done you may want to review the guide for this. http://www.sevenforums.com/tutorials...ndows-7-a.html