I was getting Instance Failure when connecting to my SQL Server Express 2005 Database in my C# ASP.NET application. Very weird.
<connectionStrings> <add name="NorthwindConnString" connectionString="Data Source=.\\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=True" providerName="System.Data.SqlClient"/> </connectionStrings>
Trial and error just taught me that the problem was "Data Source=.\\SQLEXPRESS;" - It's the double \\. That's an escape sequence in C#. Everything worked when I switched the connection string to .\SQLEXPRESS.