Windows 8 administrative shares
The default Windows 8 (and 7 for that matter) configuration rejects administrator credentials for administrative shares.
To enable administrative shares add this registry key (create a .reg file with this content and merge into the registry):
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
"LocalAccountTokenFilterPolicy"=dword:00000001