Windows

The C$ share to my knowledge cannot be removed. It is used for administrative purposes. However, you should go into the security tab for your root folder (usually C:\) and deny everything for "everyone". then you can go into your local security policy and set the security policy to lockt the system down further. see http://support.microsoft.com/default.aspx?scid=kb;en-us;823659 for information on specific security policy items. Be Careful! I was fiddling with local security policy one time and ended up making it so that nobody had access to anything.

Reg hack to disable administrative shares:
http://www.jsifaq.com/SUBA/TIP0000/RH0096.HTM

Hi,
First of all, the admin shares (C$, D$, Admin$, etc) are not available to every one, only to a user who know the Local\DomainAdmin username and password.
I agree that this can be a security problem, but you must consider carefully if you need these shares or not. For example a corporate antivirus (Symantec) require these shares in order to install remote the clients or other third-party programs like backup programs, etc.

First answer gave you the solution to disable these default shares. In addition to this, you can use a login script to merge automaticaly these registry key from a network location/share to local registry on each machine.

Of course backup a copy of your registry first.

FOR WORKSTATION

Default, Admin Shares enabled:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"AutoShareWks"=dword:00000001

Change to:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"AutoShareWks"=dword:00000000

FOR SERVER

Default, Admin Shares enabled:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"AutoShareServer"=dword:00000001

Change to:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"AutoShareServer"=dword:00000000