SP6 upgrade failure….

Event Viewer Event ID: 5702

“NetLogon service could not add the RPC interface The service was terminated. The Following error occurred:
The Authentication service is unknown.”

This occurred while trying to do a Service Pack Upgrade to SP6a.

The result is that replication of administrative services is no longer taking place.

Does anyone know how to resolve this problem without having to rebuild the PDC?

Thanks in advance.