Found a kb article that resolved a perplexing problem for us today. A Vista 64-Bit Ultimate edition PC was unable to join the domain. The error message stated a problem with RPC. This usually points to the local firewall but in this case it was ISA and a hotfix is needed to resolve it. This hotfix is available from the download center. No call to PSS required!
The kb article id is 917903; last updated March 15, 2007.
You cannot join a computer that is running a 64-bit version of Windows Vista to a Windows domain on which ISA Server 2004 is configured as a firewall
Consider the following scenario. You have a Windows domain on which Microsoft Internet Security and Acceleration (ISA) Server 2004 is configured as a firewall. You try to add to the domain a client computer that is running a 64-bit version of Windows Vista. However, you receive an "RPC Server unavailable" error message on the client computer. Additionally, the computer is not added to the domain.Note This problem occurs primarily in a Microsoft Windows Small Business Server 2003 (Windows SBS) domain.
This problem occurs because 64-bit Windows Vista client computers add a third context element structure to a remote procedure call (RPC) bind call. However, the ISA Server RPC application filter drops this bind call as an incorrect RPC bind packet.