function ajax() {
 if (typeof XMLHttpRequest != "undefined")
    return new XMLHttpRequest();
 else if (window.ActiveXObject){
    var versoes = ["MSXML2.XMLHttp.5.0",
    "MSXML2.XMLHttp.4.0", "MSXML2.XMLHttp.3.0",
    "MSXML2.XMLHttp", "Microsoft.XMLHttp", 
	"Mozilla", "FireFox", "Opera 6"
    ];
 }
 for (var i = 0; i < versoes.length; i++){
    try{
        return new ActiveXObject(versoes[i]);
    }catch (e) {}
 }
 throw new Error("Seu browser nao suporta AJAX");
}