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Symptoms
When you try to install an instance of MicrosoftSQL Server 2005 on a computer that has an instance of Microsoft SQL Server 2008 already installed, you may receive the following error message:

An instance with the same name is already installed on this computer. To proceed with SQL Server Setup, provide a unique instance name.
Resolution
This problem may occur if one of the following conditions is true:The existing instanceof SQL Server 2008 is a unique named instance. However, you try to install SQL Server 2005 as the default instance.The existing instance of SQL Server 2008 is a named instance, and you use the same name when you tryto install SQL Server 2005 as a namedinstance.