Code:

            this.comboServer.DropDown += new System.EventHandler(this.comboServer_DropDown);

        private void comboServer_DropDown(object sender, EventArgs e)
        {
            comboServer.Items.Clear();
            System.Data.Sql.SqlDataSourceEnumerator instance = null;
            System.Data.DataTable dataTable = null;
            try {
                instance = System.Data.Sql.SqlDataSourceEnumerator.Instance;
                dataTable = instance.GetDataSources();
                foreach (System.Data.DataRow row in dataTable.Rows) {
                    comboServer.Items.Add(row["ServerName"]);
                }
            } catch(Exception) {
            } finally {
                try { if(dataTable != null) dataTable.Dispose(); } catch(Exception) { }
            }
        }