I recently installed ISA Server 2006 and was unable to add the second server to the array. While investigating the issue, I could see RPC failures in the log. It was showing that the traffic was being allowed by one of the system rules, but the server failed to connect. I was able to telnet to the target domain controller on port 135 with the Microsoft Firewall disabled, but as soon as the firewall was running this communication would quit functioning. I was able to overcome the issue by changing the following registry keys:
HKLM\SYSTEM\CurrentControlSet\Services\TCPIP\Parameters
EnableRSS Change to 0
EnableTCPA Change to 0
