Under 4.0 (no service packs) it is possble to mount drive C: (and any other
drive) R/W if you have a user account.  Note that this is the default
installation and therefore it is not a bug but a missconfiguration:

smbclient '\\ntserver\c$' -U user

should give you the smb> prompt.

Installing SP2 removes these defaults.

-Yiorgos.