ok ho risolto, ora funge!

inputField=nome_del_form
codice:
function refreshPreview(inputField,outputField,httpUrl) 
{
	var myText = ""; 
	for (i=0;i<document.getElementById(inputField).length;i++)
 	  {
		 myText = myText+document.getElementById(inputField).elements[i].name+"="+escape(document.getElementById(inputField).elements[i].value);
	  	 if (i<document.getElementById(inputField).length) {myText = myText+"&";}
	  }
	
	if(window.XMLHttpRequest) {
		xmlhttp = new XMLHttpRequest();
	} else if(window.ActiveXObject) {
		xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
	} else {
		return false;
	}
	xmlhttp.open("POST", httpUrl, true);
	xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
	xmlhttp.setRequestHeader("Content-length",myText.length);
	xmlhttp.setRequestHeader("Connection","close");
	xmlhttp.onreadystatechange=function() {
		if (xmlhttp.readyState == 4) { 

			document.getElementById(outputField).innerHTML = xmlhttp.responseText;
		}
	}
	
	xmlhttp.send(myText);
}