AJAX,为什么我的xmlhttp.status == 0 ??

I am new to AJAX and I am following the tutorial from w3cSchool.com .

I basely set up a info.txt file (dummy data) on my desktop and run a html page on my desktop (i.g. under same folder) to get the text, but I end up with nothing. When I check out my xmlhttp.status, it returns with 0 value which doesn't make sense to me (the value should be either 200 or 404)

Please help me up, thanks a lot!

here is my html code:

<!DOCTYPE html>
<html>
<head>
<script>
function loadXMLDoc()
{
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("myDiv").innerHTML=xmlhttp.responseText;
    }
   else
  {
    document.getElementById("myDiv").innerHTML=xmlhttp.status;
  }
  }
xmlhttp.open("GET","info.txt",true);
xmlhttp.send();
}
</script>
</head>
<body>

<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">Change Content</button>

</body>
</html>

here is my info.txt code:

<p>123</p>
<p>AJAX is a technique for creating fast and dynamic web pages.</p>