In dired you can only view directories, and since no directory exists which contains your drive letters, you can't see a list of them.
To do this you'd have to write an emacs-lisp extension for dired.
AFAIK there's no existing extension, however, a call to wmic can give you a listing of drive letters and volume names, which would be a good starting point.
The wmic command:
wmic logicaldisk get caption,drivetype,providername,volumename
Calling it from emacs-lisp and getting the result as a string.
(let (sh-output volumes)
(setq sh-output (shell-command-to-string "wmic LogicalDisk get Caption,DriveType,ProviderName,VolumeName"))
)
Will give you a list of the volumes (DriveType : 3 = HDD, 4 = Network Mapping, 5 = Optical.)
However, you can't get dired to recognize a buffer with this output, so you'd need to create a major mode for browsing windows volumes, which would show this listing and bind RET to find the drive letter on the current line and do a dired at it's root.
If you just want the drive letters listed...
(let (sh-output volumes)
(setq sh-output (shell-command-to-string "wmic LogicalDisk get Caption"))
)
Will do that.