If you mean the prompt shown in the cmd window, it's a variable called
"PROMPT". By default it's set to $p$g which results in showing the full
path to the current location with the greater than symbol. But I don't see
anyway to get rid of the drive letter.
If you're wanting to find out the drive letter that is passed to a batch
file at the command line then that's a different story. And although I'm
sure it can be done, I don't have time right now to get to it. If I find
time later, I'll see what I can come up with.