Inserting Data - SQL Error 40 (VS2005 Express / SQL Express 2005)

Hello,
For the past few days, I've been stuck on this particular error and I hope somebody can help me. Whenever I try to connect to my database using code, I get the following error:

"An error has occurred while establishing a connection to the server.  When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) "

I did enable TCP/IP settings under the SQL Surface Area Configuration and restart the SQL server. The problem has not disappeared.

When I use the SQLDataSource tool along with the GridView tool from the Design View in VStudio, I am able to display the data from my database without any problems. Yet, when I attempt to connect using code to insert some data into my database, I am running into the above error.

My connection string looks like the following:

<connectionStrings><add name="testString" connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Customers.mdf;Integrated Security=True;User Instance=True"providerName="System.Data.SqlClient" /></connectionStrings>

And my code for inserting values into the database looks like the following:

SqlConnection con = null;

SqlCommand cmd = null;

try {

con = new SqlConnection("Server=localhost;Database=Customers.mdf;Trusted_Connection=True;");

string sqlCmd = "INSERT INTO Table (CustomerID, Name, City, State, ZipCode) VALUES ('" + nameTxt.Text + "', '" + cityTxt.Text + "', '" + stateTxt.Text + "', '" + zipTxt.Text + "');";

cmd = new SqlCommand(sqlCmd, con);

con.Open();

} finally {

// foo

}