Connecting to SQL Server 2008 on Localhost

I've written an ASP.NET website which needs to pull data from an SQL Server 2008 instance installed on the same machine.
Using the connection string:
Data Source=MYPCNAME\SQLEXPRESS;Initial Catalog=MyDatabase;Integrated Security=SSPI; the website will connect fine when I run it through debugging mode in Visual Studio 2010, but when I install as an application in IIS and try to run it through a browser without Visual Studio, the program runs but will not connect to SQL Server.
Any advice greatly appreciated.
Kind regards, Ben.
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