While attempting to install SQL Server 2008 Developer Edition on my laptop I failed on the express tools validation. Forgotten that I had even installed the express edition I immediately uninstalled and rebooted my machine. To my dismay I was blocked at the exact same point, having successfully completely uninstalled the SQL server 2005 express edition from my laptop. Seeing that it was in fact gone I was a bit perplexed and thought possibly it was a bug in the installation process. I began digging through the install logs confirming that setup thought I had the Express Tools installed as shown below
upon further investigation I found a reference to the registry key SOFTWARE\Microsoft\Microsoft SQL Server\90\Tools\ShellSEM
Slp: Initializing rule : SQL Server 2005 Express tools
Slp: Rule applied features : SSMS;Adv_SSMS
Slp: Rule is will be executed : True
Slp: Init rule target object: Microsoft.SqlServer.Configuration.SetupExtension.Sql2005SsmsExpressFacet
Slp: Sco: Attempting to create base registry key HKEY_LOCAL_MACHINE, machine
Slp: Sco: Attempting to open registry subkey
Slp: Sco: Attempting to open registry subkey SOFTWARE\Microsoft\Microsoft SQL Server\90\Tools\ShellSEM
Slp: Rule ‘Sql2005SsmsExpressFacet’ detection result: SQL 2005 Ssms EE installed=True
Slp: Evaluating rule : Sql2005SsmsExpressFacet
Slp: Rule evaluation done : Failed
Slp: Rule evaluation message: The SQL Server 2005 Express Tools are installed. To continue, remove the SQL Server 2005 Express Tools.
Checking the registry I find that the key does indeed exist and in fact was added by the install of Redgate SQL Prompt 3.5 as shown below
I exported the offending registry key, deleted it, installed SQL server 2008 without an issue and then imported the sql promt key back into the registry.