Found this:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224370
SYMPTOMS
When you perform any of the following server-based tasks, you may receive an "RPC server is unavailable" error message:
Replication
Winlogon
Enable trusted relationships
Connect to domain controllers
Connect to trusted domains
User authentication
NOTE: You may also get the error "RPC server is unavailable" running Dcpromo on a member server. This can occur if you have only one DC and it does not have file and printer sharing enable on its network card.
CAUSE
This issue can occur for any of the following reasons:
The RPC service may not be started.
You are unable to resolve a DNS or NetBIOS name.
An RPC channel cannot be established.
RESOLUTION
To resolve this issue:
Click Start, click Run, type the following line in the Open box, and then click OK:
net start rpcss
Test to see if this resolves the issue. If the issue still occurs, continue to the next step.
Click Start, point to Programs, point to Accessories,and then click Command Prompt.
At the command prompt, type ping servername, where servername is the server, NetBIOS, DNS, or GUID name that you want to test for connectivity.
If there is a connection issue with one of these computers, contact your network administrator to resolve the issue. If the issue still occurs, continue to the next step.
Use the Netdiag tool that is included in the Microsoft Windows Support Tools (included on the Windows CD-ROMs) to determine if the domain controller is working correctly. You can perform a network trace using the MSRPC, DNS, NBT, LDAP, or TCP protocols.
If there is an issue with the domain controller, contact your network administrator to resolve the issue. If the issue still occurs, continue to the next step.
Use the Netdom tool that is included in the Windows Support Tools to verify network trust relationships and reset or establish a connection to a server.