By default, Windows 7 will not allow access to the C$ administrative share, so you’ll have trouble getting BackupPC to work. To solve this, run up Registry Editor and set the following registry key;
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\LocalAccountTokenFilterPolicy
To the REG_DWORD (32-bit) value of 1.
Source: http://www.howtogeek.com/howto/windows-vista/enable-mapping-to-hostnamec-share-on-windows-vista/