<script type="text/javascript"> 
 	 	var xmlDoc=null; 
 	 	 try{ 
 		 	xmlDoc=new ActiveXObject("Microsoft.XMLDOM"); 
 		 }catch(e){ 
 		 	try { 
 				xmlDoc = document.implementation.createDocument("", "", null); 
 			}catch(ee){ee.message} 
 		 } 
 		 xmlDoc.async=false; 
 		 xmlDoc.load("XML_001.xml"); 
 		 var username=xmlDoc.getElementsByTagName("username")[0].childNodes[0].nodeValue; 
 		 document.write(username); 
 	  </script> 
 

 <script type="text/javascript">
    var xmlDoc = null;
    var xml = "XML_001.xml";
    if (window.ActiveXObject) {
        xmlDoc = new ActiveXObject('Microsoft.XMLDOM');
        xmlDoc.async = false;
        xmlDoc.load(xml);
    } else if (document.implementation && document.implementation.createDocument) {
        try{
            xmlDoc = document.implementation.createDocument('', '', null);
            xmlDoc.async = false;
            xmlDoc.load(xml);
        } catch(e){
            var xmlhttp = new window.XMLHttpRequest();
            xmlhttp.open("GET",xml,false);
            xmlhttp.send(null);
            xmlDoc = xmlhttp.responseXML.documentElement;
        }
    }
    var username=xmlDoc.getElementsByTagName("username")[0].childNodes[0].nodeValue;
</script>

------解决方案--------------------
JScript code
function loadXML(url)
{
    var xmlhttp;
    if( window.XMLHttpRequest )
        xmlhttp = new XMLHttpRequest();
    else{
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.open('GET', url);
    xmlhttp.onreadystatechange = function(){
        if( xmlhttp.readyState==4 && xmlhttp.status==200 ){
            return xmlhttp.responseXML;
        }
    }    
    xmlhttp.send();
}