var xmlHttp;

function cerca() {
    var strElementById = "selectProvincia"
    var valorecampo = document.getElementById(strElementById).options[document.getElementById(strElementById).selectedIndex].value;
    // per l'oggetto nativo XMLHttpRequest
    if (window.XMLHttpRequest) {
        xmlHttp = new XMLHttpRequest();
        xmlHttp.onreadystatechange = ricevi;
        xmlHttp.open("GET", "includes/ajax_prov_comuni.asp?qrystr=" + valorecampo, true);
        xmlHttp.send(null);
        // per IE
    } else if (window.ActiveXObject) {
        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        if (xmlHttp) {
            xmlHttp.onreadystatechange = ricevi;
            xmlHttp.open("GET", "includes/ajax_prov_comuni.asp?qrystr=" + valorecampo, true);
            xmlHttp.send();
        }
    }
}

function ricevi()
{
    if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
    {
        document.getElementById("boxComuni").innerHTML=xmlHttp.responseText
    }
}