codice:
<html>
<head>

        <style type="text/css">
                body{
                        font-family:verdana;
                        font-size:8pt;
                }
                input.stato{
                        display:none;
                        border-color:#fff;
                        border:0px;
                        color:#0000aa;
                }
         </style>

        <script type="text/javascript">
                var req;
                var lastValue = [];
                
                function createXmlRequest(){
                        // ...
                        return new XMLHttpRequest();
                }

                function recorder(ido){
                        alert(ido);
                        var valore = document.getElementById(ido).value;
                        if(lastValue[ido]){
                                if(lastValue[ido] == valore){
                                        // nothing
                                }else{
                                        lastValue[ido] = valore;     
                                        getValue(valore, ido);                                
                                }
                        }else{
                                lastValue[ido] = valore;
                                getValue(valore, ido);
                        }
                }
                
                document.getElementById("uname").onblur=recorder("uname");
                document.getElementById("pwd").onblur=recorder("pwd");      
          
                function takeResponse(){    
                        if(req.status==200){
                                alert("c");
                                var res = req.responseText;
                                document.getElementById("stato").style.display='block';
                                document.getElementById("stato").innerHTML=res;  
                                alert(res);
                                //delete req;
                        }
                }
                
                function getValue(valore, idobj){
                        if(req==null)
                                req = createXmlRequest();
                        alert("il primo");
                        req.open("GET","elab.php?field="+idobj+"&value="+valore, true);
                        req.send(null);
                        req.onreadystatechange = takeResponse;
                        alert("pre");
                }
        </script>

</head>
<body>

<div id="stato" class="stato">

</div>
<form action="login.php" id="login">
        <label for="uname">Username</label>
        <input type="text" name="username" id="uname" />

        <label for="pwd">Password</label>
        <input type="password" name="passw" id="pwd" />

        <input type="submit" name="sent" value="Login" id="sub" />
</form>


</body>
</html>
Tutta qui la pagina.
Gli alert() servono solo per determinare il "percorso" dell'applicazioncina.