Error message when you connect to a named instance of SQL Server on a client computer that is running Windows Vista or Windows Server 2008: "Specified SQL server not found" or "Error Locating Server/Instance Specified"
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Source: Knowledge Base
Product: Microsoft SQL Server 2005 Enterprise Edition & SQL Server 2005 Standard Edition
Notification Contents: New and Major Modifications
Error message when you connect to a named instance of SQL Server on a client computer that is running Windows Vista or Windows Server 2008: "Specified SQL server not found" or "Error Locating Server/Instance Specified"
http://support.microsoft.com/kb/944390/en-US