Symptoms
You upgrade a clustered instance of Microsoft SQL Server 2005 to Microsoft SQL Server 2008. Then, you uninstall the clustered instance of SQL Server 2008 and all the remaining components of SQL Server 2005. When you try to reinstall SQL Server 2005, you receive the following error message:

This application has failed to start because the application configuration is incorrect. Reinstalling the application may fix this problem.
Resolution
This problem occurs because the runtime components of Microsoft Visual C++ 2005 Service Pack 1 (SP1) are uninstalled when you uninstall SQL Server 2008. When you uninstall SQL Server 2008, the cluster resource files (.dll files) are not uninstalled and are shared by the later SQL Server 2005 installation. However, the runtime components of Visual C++ 2005 SP1 are required for you to use these .dll files.
When you install a clustered instance of SQL Server 2005, these .dll files are used to cluster the instance. However, SQL Server 2005 includes only the release version of the Visual C++ 2005 runtime. Therefore, an error occurs when the Setup program tries to load these .dll files.