/*
调用ajax：
url：要调用的页面
callBack：调用成功后，回调的方法
*/
function CallAjax(url, callBack)
{
    if(window.XMLHttpRequest)
    {
        req = new XMLHttpRequest();
    }
    else if(window.ActiveXObject)
    {
        req = new ActiveXObject("Microsoft.XMLHTTP");
    }

    if(req)
    {
	    req.open("GET", url, false); 
	    req.onreadystatechange = function CallByAjax()
	    {
            if (req.readyState == 4 && req.status == 200) 
            {
                callBack(req.responseXML);
            }
	    }
    }
    req.send(null); 
}