I m using win7 and VS2008 and sql server 2005

I have an application of silver light with WCF ,(i need to bind grid with database records) but its giving me an excpetion while opening conenction to database..Although i make changes in sql server Express to connect locally and remotely (asi m using WFCF). my connection string in web.config is
Code:
	<connectionStrings>
		<add name="NorthwindConnectionString" connectionString="Data Source=(IRAM-PC);Database=Northwind;User Id=sa;Password=sa"  providerName="System.Data.SqlClient" />
      </connectionStrings>
other piece of code is here
Code:
using System.Data;
using System.Data.SqlClient;
 
using System;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Activation;
using System.Collections.Generic;
using System.Text;

namespace SilverlightWithWCFService.Web
{
    [ServiceContract(Namespace = "")]
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
    public class SampleService
    {
        [OperationContract]
        public List<Customer> CustomerList()
        {
            string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString;
            var custList = new List<Customer>();
            using (SqlConnection conn = new SqlConnection(nwConn))
            {
                const string sql = "SELECT TOP 10 CustomerID, CompanyName, ContactName FROM Customers";
                conn.Close();
                conn.Open();
                using (SqlCommand cmd = new SqlCommand(sql, conn))
                {
                    SqlDataReader dr = cmd.ExecuteReader(
                        CommandBehavior.CloseConnection);
                    if (dr != null)
                        while (dr.Read())
                        {
                            var cust = new Customer
                            {
                                CustomerName = dr.GetString(0),
                                CompanyName = dr.GetString(1),
                                ContactName = dr.GetString(2)
                            };
                            custList.Add(cust);
                        }
                    return custList;
                }
            }
        }

        // Add more operations here and mark them with [OperationContract]
    }
}
giving me an exception on the line conn.Open();

any help!!