I struggled with this for hours after being hacked via FTP. Some idiots created some directories with blank (ASCII 32) directory names. Not just directories with blanks IN the names, but the entire directory name consisted of one blank or space.

I attempted to delete using Windows Explorer, command prompt, FTP clients, and other things with no success. Each try would get an error like "cannot read from device" or "invalid directory name". Renaming didn't work either.

The solution is remarkably easy: go to the command prompt (run: cmd), change to the directory just above the blank-named one, and type:
DIR /X /P
When you do, you'll see the Windows-assigned short file names along with the regular names. When you see the one with a blank for the regular directory name, you'll see something like 0200~1 as the short name. Now, from the same place you currently are, type:
RD 0200~1
or whatever was shown as the name for your problem directory.

Thanks to John Stephenson for figgering this out for all of us.