If on the older version of terminal services you get this error on login, and you can connect to the registry and remote services, however RDP fail with this:
"The system cannot log you on due to the following error: The RPC server is unavailable"
Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server
Create the following key:
Name: IgnoreRegUserConfigErrors
Type: DWORD
Value : 1