Could not find installable ISAM.

 
 private string constring = "Provider=Microsoft.Jet.OLEDB.4.0;Jet OLEDB:Database Password=admin;UserID=Admin;Data Source=" + Application.StartupPath + "\\db\\IRNdb.mdb";        private bool CheckUser(string uname,string pwd)
        {
            string csql = "select * from roleT where uid='" + uname.ToString() + "' and pwd='" + pwd.ToString().Trim() + "'";
            OleDbConnection connection = new OleDbConnection(constring);
            connection.Open();
            OleDbCommand ocmd = new OleDbCommand(csql, connection);
            OleDbDataReader dr = ocmd.ExecuteReader();
            if (dr.Read())
            {
                connection.Close();
                return true;
            }
            else
            {                connection.Close();
                return false;
            }
        }
请各位大侠帮忙看看，在执行到 connection.Open()时，就报错：“Could not find installable ISAM.”
请问怎么解决，比较紧急，请大家帮忙看看，谢谢！