prendendo spunto dalla guida Html.it ti do il codice da me semplificato:

Codice PHP:
function caricaAjax(Cosa,Dove,Come) {
    var 
elemento document.getElementById(Dove);
    var 
usaLink true;
    if(!
Come) var Come="get";
    
browserUtente navigator.userAgent.toUpperCase();
    if(
typeof(XMLHttpRequest) === "function" || typeof(XMLHttpRequest) === "object")                ajax = new XMLHttpRequest();
    else if(
window.ActiveXObject && browserUtente.indexOf("MSIE 4") < 0) {
        if(
browserUtente.indexOf("MSIE 5") < 0)                                                                                             ajax = new ActiveXObject("Msxml2.XMLHTTP");        
        else                                                                                                                                                                     
ajax = new ActiveXObject("Microsoft.XMLHTTP");
        }
    if(
ajax) {
        
usaLink false;
        
ajax.open(ComeCosatrue);
        
ajax.setRequestHeader("connection""close");
        
ajax.onreadystatechange = function() {
            if(
ajax.readyState === readyState.COMPLETATO) {
                if(
statusText[ajax.status] === "OK")    elemento.innerHTML ajax.responseText
                else {                                                                 
elemento.innerHTML "Impossibile effettuare l'operazione richiesta.
"
;
                                                                                            
elemento.innerHTML += "Errore riscontrato: " statusText[ajax.status]; }  
            }}  
        
ajax.send(null);   
        } 
    return 
usaLink;