var xmlHttp = createXmlHttpRequestObject();

function createXmlHttpRequestObject(){
    var xmlHttp;
    try{
        xmlHttp = new XMLHttpRequest();
    }catch(e){
        var XmlHttpVersions = new Array('MSXML2.XMLHTTP.6.0',
            'MSXML2.XMLHTTP.5.0',
            'MSXML2.XMLHTTP.4.0',
            'MSXML2.XMLHTTP.3.0',
            'MSXML2.XMLHTTP',
            'Microsoft.XMLHTTP');
        for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++){
            try{
                xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
            }catch (e) {}
        }
    }
    if (!xmlHttp){
        alert("Erro ao criar o XMLHttpRequest.");
    }
    else{
        return xmlHttp;
    }
}