Ajax xml

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script type="text/javascript">
var xmlHttp;
function createXMLHttpRequest()
{
	if(window.ActiveXObject)
	{
		xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
	}
	else if(window.XMLHttpRequest)
	{
		xmlHttp = new XMLHttpRequest();
	}
}
function startRequest()
{
	createXMLHttpRequest();
	xmlHttp.onreadystatechange = handleStateChange;
	xmlHttp.open("GET","simpleResponse.xml",true);
	xmlHttp.send(null);
}
function handleStateChange()
{
	if(xmlHttp.readyState == 4)
	{
		if(xmlHttp.state == 200)
		{
			alert("The server replied with: " + xmlHttp.responseText);
		}
	}
}
</script>
</head>

<body>
	<form action="#">
		<input type="button" value="Start Basic Asynchronous Request" onclick="startRequest()" />
	</form>
</body>
</html>

simpleResponse.xml
文件里面要怎么写
为什么警告框跳不出来