Think this would work, but you should probably test it just to make sure. I'm far from a regex expert, so there's probably a better way to do it. Perhaps someone will post it.
win_path = re.sub(r"/([A-Za-z])/(.+)", r"\1:/\2", nix_path)
win_path = re.sub(r"/", r"\\", win_path)
It converts the posted path to