Easy. Plug in the USB you want to allocate a specific letter.
Right-click on (My) Computer (if you can find it ha ha) and choose Manage.
Then in the next window, click on Disk Management.
In the lower window, you'll see all attached drives, including ones that haven't been allocated a drive letter. This scared the beejeesus out of me when I downgraded to W7 and W8, but don't worry.
Simply right click on the drive concerned and choose "Change Drive Letter and Paths". (Re)assign the letters you want.
If an internal drive already has the letter you want to allocate to the USB drive eg the DVD, just change that one to another, then give the USB drive the letter.
Once the letters are assigned to the USB drives, they will stick on that machine unless another drive takes it before you insert it eg a phantom drive with hidden files or folders.