I was trying to install a named instance of SQL Server 2008 R2 on my machine, and I got an error telling me that the installation cannot continue because the SQL 2005 Express tools are installed. I had never installed SQL 2005 on my machine, and after some searching I found that the error is due to the following entry in the registry:
SOFTWARE\Microsoft\Microsoft SQL Server\90\Tools\ShellSEM
Well, it turned out that this key had something to do with me installing some RedGate software on my machine previously.
Here is how you fix it: Backup the registry key, delete it, continue with the SQL Server install, restore the registry key.
