Проблемка с AJAX

var div_result="";
var urlString="";
var StringSending;
var result;

function SetParam(_div_result, _urlString, _StringSending){
    showloadming(true);
    div_result=document.getElementById(_div_result);
    urlString=_urlString;
    StringSending="param="+_StringSending;
    ajax();
}

function ajax() {
    if (window.XMLHttpRequest) {
        req = new XMLHttpRequest();
        req.open("POST", urlString, true);
    req.setRequestHeader("Method", "POST " + urlString + " HTTP/1.1");
    req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=windows-1251");
    req.onreadystatechange = processReqChange;
    req.send(StringSending);
    } else if (window.ActiveXObject) {
        req = new ActiveXObject("Microsoft.XMLHTTP");
        if (req) {
            req.open("POST", urlString, true);
            req.setRequestHeader("Method", "POST " + urlString + " HTTP/1.1");
        req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=windows-1251");
        req.onreadystatechange = processReqChange;
        req.send(StringSending);
        }
    }
}

function processReqChange() {
    ab = window.setTimeout("req.abort();", 5000);
    if (req.readyState == 4) {
        clearTimeout(ab);
        if (req.status == 200) {
            showloadming(false);
            result=req.responseText;
            div_result.innerHTML=req.responseText;
            showloadming(false);
        } else {
            alert("It was not possible to obtain the data:\n" + req.statusText);
        }
    }
}