window.onload = initAll; 
  
 var xmlhttp; 
 var time = 500; 
  
 function initAll() { 
 	refresh(); 
 } 
  
 function refresh() { 
 	if (window.XMLHttpRequest) { 
 		// 针对FireFox,Mozillar,Opera,Safari,IE7,IE8 
 		xmlhttp = new XMLHttpRequest(); 
  
 		if (xmlhttp.overrideMimeType) { 
 			xmlhttp.overrideMimeType("text/xml"); 
 		} 
 	} else if (window.ActiveXObject) { 
 		// 针对IE5,IE5.5,IE6 
 		var activexName = [ "MSXML2.XMLHTTP", "Microsoft.XMLHTTP" ]; 
  
 		for ( var i = 0; i  < activexName.length; i++) { 
 			try { 
 				xmlhttp = new ActiveXObject(activexName[i]); 
 				break; 
 			} catch (e) { 
 			} 
 		} 
 	} 
 	xmlhttp.onreadystatechange = callback; 
 	xmlhttp.open("GET", "checkNewMessage", true); 
 	xmlhttp.send(null); 
 } 
 function blink() { 
 	var spanNode = document.getElementById("result"); 
 	spanNode.style.visibility = (spanNode.style.visibility == "hidden") ? "visible":"hidden"; 
 	setTimeout("blink()", time); 
 } 
  
 function callback() { 
 	if (xmlhttp.readyState == 4) { 
 		if (xmlhttp.status == 200) { 
 			var responseText = xmlhttp.responseText; 
 			var spanNode = document.getElementById("result"); 
 			spanNode.innerHTML = responseText; 
 			setTimeout("refresh()", 120000); 
 			setTimeout("blink()", time); 
 		} 
  
 	} 
 } 
  
  
 

<span id="result" style="color:red; visibility:hidden"></span>

xmlhttp.open("GET", "checkNewMessage?_dc="+new Date().getTime(), true);