关于IE8 创建XMLHttpRequest 的问题

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<%

	String path = request.getContextPath();

	String basePath = request.getScheme() + "://"

			+ request.getServerName() + ":" + request.getServerPort()

			+ path + "/";

%>



<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

	<head>

		<base href="<%=basePath%>">



		<title>My JSP 'MyJsp.jsp' starting page</title>



		<meta http-equiv="pragma" content="no-cache">

		<meta http-equiv="cache-control" content="no-cache">

		<meta http-equiv="expires" content="0">

		<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">

		<meta http-equiv="description" content="This is my page">

		<!--

	<link rel="stylesheet" type="text/css" href="styles.css">

	-->

		<script type="text/javascript">

function showHint() {

	var xmlhttp;

	

	if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari

		xmlhttp = new XMLHttpRequest();

	} else {// code for IE6, IE5

		xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");

	}

	xmlhttp.onreadystatechange = function() {

		if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {

			document.getElementById("txtHint").innerHTML = xmlhttp.responseText;

		}

	}

	xmlhttp.open("GET", "web/enterprise/pagetag.action", true);

	xmlhttp.send();

	setTimeout("showHint()",2000);

}

</script>

	</head>



	<body onload="showHint()">

	 

		<p>

		 

			<span id="txtHint"></span>

		</p>

	</body>

</html>

最后一句 setTimeout("showHint()",2000);
是用来设置刷新的时间，这种写法在火狐、google浏览器中都可以运行，可以刷新。
但是在IE8中可以出结果，但是就是不能刷新，当使用IE8的兼容模式时菜可以自动的刷新，请问是什么原因导致的呢？