Microsoft.XMLHTTP pagina remota

[-AA-]

Sto facendo un form con "controlli istantanei, ma...su firefox funziona...mentre su IE no :-(
questa la demo

<link rel="stylesheet" type="text/css" href="style.css" />
<script src="ajaxforms.js" type="text/javascript" language="javascript"></script>
<form action="" method="post">
<table class=ajform>

<tr>
<td class="title">
Username (*)
</td>
<td>
<input type="text" name="username" value="" size="20" OnChange="CheckField(this)" tabindex="1"></td><td><div id="username" class="message"></div></td>
</tr>

</table>
</form>

qusto il contenuto del file ajaxforms.js

var AJAXForms = false;
var LastField = null;
var isIE = false;
// on !IE we only have to initialize it once
if (window.XMLHttpRequest) {
	AJAXForms = new XMLHttpRequest();
}

function CheckField(field) {

	if (window.XMLHttpRequest) {
	// branch for IE/Windows ActiveX version
	} else if (window.ActiveXObject) {
		AJAXForms = new ActiveXObject("Microsoft.XMLHTTP");
	}

  AJAXForms.onreadystatechange = processChange;
  AJAXForms.open("GET", "check_form.php?op=ajax&field=" +field.name+"&value=" + field.value);
  LastField = field.name;
  AJAXForms.send(null);
}

function processChange() {
  if (AJAXForms.readyState == 4) { 
  
    var  res = document.getElementById(LastField);
    res.innerHTML = AJAXForms.responseText;
    res.style.visibility = "visible";
    
  }
}

che devo fare per farlo girare anche su IE :-( ?