function list_dados( valor )
{  http.open("GET", "result.php?id=" + valor, true);  
	http.onreadystatechange = handleHttpResponse;
	http.send(null);
}
//

function handleHttpResponse()
{  
	campo_select = document.forms[0].bairro;
	  if (http.readyState == 4) { 
	  	   campo_select.options.length = 0;   
		   //acrescentei essa linha;
		    campo_select.options[0] = new Option("Todos os Bairros", '0');
			//
		   results = http.responseText.split(",");   
		    for( i = 0; i < results.length; i++ )  
  {       string = results[i].split( "|" );   
    campo_select.options[i+1] = new Option( string[0], string[1] );
    }  }}
function getHTTPObject()
 {var req;try { if (window.XMLHttpRequest) {  req = new XMLHttpRequest();  if (req.readyState == null) {   req.readyState = 1;   req.addEventListener("load", function () {   req.readyState = 4;   if (typeof req.onReadyStateChange == "function")    req.onReadyStateChange();   }, false);  }  return req; }

 if (window.ActiveXObject) {  var prefixes = ["MSXML2", "Microsoft", "MSXML", "MSXML3"];
  for (var i = 0; i < prefixes.length; i++) {   try {    req = new ActiveXObject(prefixes[i] + ".XmlHttp");    return req;   } catch (ex) {};
  } }} catch (ex) {}
alert("XmlHttp Objects not supported by client browser");}var http = getHTTPObject();
// Logo após fazer a verificação, é chamada a função e passada // o valor à variável global http.