var http_request = false;
function ajax (url, dados, metodo, funcao) {
http_request = false;
if (!dados) dados = null;
if (!metodo) metodo = 'GET';
if (!funcao) funcao = null;

if (window.XMLHttpRequest) { // Mozilla, Safari,...
	http_request = new XMLHttpRequest();
	if (http_request.overrideMimeType) http_request.overrideMimeType('text/xml');
} else if (window.ActiveXObject) { // IE
	try {
		http_request = new ActiveXObject("Msxml2.XMLHTTP");
	} catch (e) {
		try {
			http_request = new ActiveXObject("Microsoft.XMLHTTP");
		} catch (e) {}
	}
}

if (!http_request) return false;

if (metodo == 'POST') http_request.setRequestHeader ('Content-Type', 'application/x-www-form-urlencoded');
http_request.onreadystatechange = funcao;
http_request.open (metodo, url, true);
http_request.send (dados);
}