What a surprise when I needed it a year ago while working in Windows XP and couldn't find it anywhere; it certainly existed in Windows 9x! Where is it? Well, it simply does not exist any more.
So, how can you delete whole directory trees from the Windows XP command line? Just use the rmdir command. Its /s option can be specified to ask it to recursively delete directories. And the /q flag comes helpful to avoid the multiple and annoying "Are you sure?" questions.
As far as I can tell, deltree ceased to exist in Windows 2000. And yes: I have been meaning to post this message for almost a year!