Could not find installable ISAM.

Forresten jeg benytter c# .NET og klassen der håndtere databaseforbindelsen ser således ud:

using System.Data;
using System.Data.OleDb;
using System.Web;
using System.Configuration;
using System.Collections.Specialized;

namespace MessageBoardObjects
{

    public class DataControl
    {

        public static DataSet GetDataSet(string SQL)
        {
            string connectionString;
            NameValueCollection appSettings;
            appSettings = System.Configuration.ConfigurationManager.AppSettings;
            //connectionString = appSettings["ConnectionString"];
            connectionString = @"PROVIDER=Microsoft.Jet.OLEDB.4.0;DATA SOURCE=C:\messageboard.mdb";
            OleDbConnection connection = new OleDbConnection(connectionString);
            connection.Open();
            OleDbDataAdapter adapter = new OleDbDataAdapter(SQL, connection);
            DataSet myData = new DataSet();
            adapter.Fill(myData);
            adapter.Dispose();
            connection.Close();
            return myData;
        }

        internal static void ExecuteNonQuery(string SQL)
        {
            string connectionString;
            NameValueCollection appSettings;
            appSettings = System.Configuration.ConfigurationManager.AppSettings;
            connectionString = appSettings["ConnectionString"];
            OleDbConnection connection = new OleDbConnection(connectionString);
            connection.Open();
            OleDbCommand myCommand = new OleDbCommand();
            myCommand.Connection = connection;
            myCommand.CommandText = SQL;
            myCommand.CommandType = CommandType.Text;
            myCommand.ExecuteNonQuery();
            connection.Close();
            myCommand.Dispose();
            connection.Dispose();
        }
    }
}

Problemet var at min connectionstring stod i web.config. Noget går galt når mit class lib kalder den. Lukker