/* Função para criar o objeto XMLHttpRequest */
function openAjax()
{
	try
	{
		var ajax = new XMLHttpRequest();
	}
	catch(e)
	{
		try
		{
			var ajax = new ActiveXObject("Msxml2.XMLHTTP");
		}
		catch(ee)
		{
			try
			{
				var ajax = new ActiveXObject("Microsoft.XMLHTTP");
			}
			catch(eee)
			{
			    var ajax = false;
			}
		}
	}
	
	return ajax;
}

var req;

function validarDados(campo, valor) {

        if(window.XMLHttpRequest) {
                req     = new XMLHttpRequest();
        }
        // Internet Explorer
        else if(window.ActiveXObject) {
                req = new ActiveXObject("Microsoft.XMLHTTP");
        }
        var url = "validacao.php?campo="+campo+"&valor="+valor;
        req.open("Get", url, true); 
        req.onreadystatechange = function() {
                if(req.readyState == 1) {
                        document.getElementById('campo_' + campo + '').innerHTML = '<font color="gray">Verificando...</font>';
                }
                if(req.readyState == 4 && req.status == 200) {   
                        var resposta = req.responseText;
                        document.getElementById('campo_'+ campo +'').innerHTML = resposta;
                }
        }
        req.send(null);
}