Do you mean the administrative C$ share? You can disable that with a
registry edit:
Deleting Default Administrative Shares for Current and Future Sessions
WARNING: If you use Registry Editor incorrectly, you may cause serious
problems that may require you to reinstall your operating system. Microsoft
cannot guarantee that you can solve problems that result from using Registry
Editor incorrectly. Use Registry Editor at your own risk.
To delete the hidden administrative shares for all root partitions and
volumes (such as C$) and the system root folder (ADMIN$) and prevent Windows
from re-creating them, add an AutoShareWks DWORD value to the following
registry key and set its value data to 0:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
The full text is found at this KB article:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q314984&sd=tech
(All usual warnings regarding the perils of Registry editing apply.)
If your C: drive is manually shared, just right-click on the drive letter in
Explorer, go to the Sharing tab and click "Do not share this drive".