Could not connect to X64 named instance

We see a couple of cases where customer could not connect to named instance on X64. They all have the following symptons.

1) Customer has no problem to connect X86 instance on the same server machine even if it's a named instance.
Customer can even connect to some of the X64 named instance, but always see problem when connect to some specific named X64 instance.
2) SQL Server is confirmed up and running. The server is listening on TCP and/or NP protocols.
3) SQL Browser is confirmed in running state.
4) Customer confirmed it's not a firewall issue. (either by disabling firewall on both client and server machine temprarily, or by adding needed firewall exceptions for application or port number)

If you see this happens on your X64 named instance, then the solution below may applies to you. We have confirmed a bug in SQL Server 2005 could cause this problem.

Let's say your X64 instance is INST1. You can find its instance ID from HKEY_LOCAL_MACHINESOFTWAREMicrosoftMicrosoft SQL ServerInstance NamesSQL, say it's MSSQL.3. Then the configuration of this instance is under 
HKEY_LOCAL_MACHINESOFTWAREMicrosoftMicrosoft SQL ServerMSSQL.3
Please check if you have the following entry in registry:
HKEY_LOCAL_MACHINESOFTWAREWow6432MicrosoftMicrosoft SQL ServerMSSQL.3

If you do have it, then it's the root cause of the problem and you should remove it to solve the problem. INST1 is an X64 instance and its configuration should not present under Wow hive. This wrong registry might come from a previous instance which you have removed but the registry is not cleaned by the SQL uninstallation for some reason.

A good news is that the issue is resolved in SQL Sever 2008. Browser will respond correctly even if there is wrong entry.

Xinwei Hong, SQL Server Protocols
Disclaimer: This posting is provided "AS IS" with no warranties, and confers no rights