function createXMLHTTP() 
{
    var ajax;
    try 
    {
        ajax = new ActiveXObject("Microsoft.XMLHTTP");
    } 
    catch(e) 
    {
        try 
    {
    ajax = new ActiveXObject("Msxml2.XMLHTTP");
    alert(ajax);
}
catch(ex) 
{
    try 
    {
        ajax = new XMLHttpRequest();
    }
    catch(exc) 
    {
        alert("Esse browser não tem recursos para uso do Ajax");
        ajax = null;
    }
}
    return ajax;
}

var arrSignatures = ["MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0",
"MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP",
"Microsoft.XMLHTTP"];
for (var i=0; i < arrSignatures.length; i++) 
{
    try 
    {
        var oRequest = new ActiveXObject(arrSignatures[i]);
        return oRequest;
    } 
    catch (oError) 
    {
    }
}
throw new Error("MSXML is not installed on your system.");
}