<!--AJAX-->
function createRequestObject() {
    var ro;
    var browser = navigator.appName;
    if(browser == "Microsoft Internet Explorer"){
        ro = new ActiveXObject("Microsoft.XMLHTTP");
    }else{
        ro = new XMLHttpRequest();
    }
    return ro;
}

var http = createRequestObject();

function validateForm(txtfield){
    var url = '/includes/validateregister.php?';
    var url2;
    var email = document.getElementById('email').value;
    url2 = txtfield+"="+email;
    url += url2;
    http.open('get', url);
    http.onreadystatechange = handleResponse;
    http.send(null);
}

function handleResponse() {
    if(http.readyState == 4){
        var response = http.responseText;
        var UPDATE = response;
        document.getElementById('nameerr').innerHTML="";
        <!--LOOP THROUGH THE RESPONSES-->
            if(UPDATE != ''){
                // document.getElementById(UPDATE[0]).className = 'error';
                document.getElementById('email').value = UPDATE;
             }else {
             	document.getElementById('nameerr').innerHTML="<br /><p style='color: #F00'>That email address is already in use</p>";
                document.getElementById('email').value = "";
                document.getElementById('email').focus();
             }
     }
}
<!--END AJAX-->