Windows has a bad habit of enabling by default some shares on a new installation. These are, for example, C$, D$ (they give access to your partition roots) … ADMIN$, IPC$
Those shares are accessible by the administrators, but they could be a useless security breach.
Even worse, if you delete them, they will come back at the next start of the Server service starts…
If you wish to permanently disable them then create the following key in the registry (if it already exists, set the value to 0) :
REG_DWORD = 0
more good informations here : http://www.petri.co.il/disable_administrative_shares.htm