function getXmlHttpRequestObject() {
    if (window.XMLHttpRequest) {
        return new XMLHttpRequest();
    } else if (window.ActiveXObject) {
        return new ActiveXObject("Microsoft.XMLHTTP");
    } else {

        alert("Your browser doesn't support the XmlHttpRequest object.");
    }
}
var Req = getXmlHttpRequestObject();

function makeRequest(url, param) {

    if (Req.readyState == 4 || Req.readyState == 0) {

        Req.open("POST", url, true);

        Req.onreadystatechange = updatePage;


        Req.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        Req.setRequestHeader("Content-length", param.length);
        Req.setRequestHeader("Connection", "close");


        Req.send(param);
    }
}

function updatePage() {

    if (Req.readyState == 4) {

        img = document.getElementById('imgCaptcha');

        img.src = 'create.php?' + Math.random();
    }
}

function getParam(theForm) {

    var url = 'mail1.php';

    var postStr = theForm.txtCaptcha.name + "=" + encodeURIComponent(theForm.txtCaptcha.value);

    makeRequest(url, postStr);
}