By default you can’t connect to the C$ or “Admin” share on Windows 7. This is a security feature designed to prevent Loopback attacks and is part of UAC. You can disable this security control without disabling UAC completely by changing (or creating, if it doesn’t exist) the LocalAccountTokenFilterPolicy 32bit DWORD with a value of 1 in the following registry location:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\
Note that this will disable UAC Remote Restrictions and enable loopback attacks on the computer.
You should then be able to connect to C$ and other admin shares on the computer.