求助,javascript动态改变数据岛的src?

<body>
<XML ID='rsstsMain' SRC='http://www.microsoft.com/china/technet/abouttn/rss/security.xml'></XML>
</div>
<script language='javaScript'>
var xmldata
function tabSelect(obj)
{  
document.all.rsstsMain.parentElement.removeChild(document.all.rsstsMain)
xmldata = document.createElement("XML");
document.body.insertBefore(xmldata);
xmldata.id = "rsstsMain";
xmldata.src = "http://dotnet.aspx.cc/Rss.aspx";
xmldata.onreadystatechange=transform;
}
var tid=null
function transform()
{
if(xmldata.readyState=="complete")
{
clearInterval(tid)
y.innerHTML="ok"
}
else
tid = setInterval("transform()",10)
}
function showXML()
{
alert(document.all.rsstsMain.xml);}
</script>
<input onclick="showXML()" type="button" value="显示文件内容">
<input onclick="tabSelect(null);" type="button" value="装载新的文件">
<div id=y></div>
</body>