var Request = null ;

if(window.XMLHttpRequest)
	{
		Request = new XMLHttpRequest();
			if(Request.overrideMimeType)
				Request.overrideMimeType('text/xml');
	} else
	if(window.ActiveXObject)
	{
		var prefixes = [ "MSXML2.XMLHttp.5.0","MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0","MSXML2.XMLHttp","Microsoft.XMLHttp"];
		
		for(var i = 0; i < prefixes.length; i++)
		{
			try
			{
				Request =  new ActiveXObject(prefixes[i]);
			} catch (e) {Request = null;}
		}
	}

if(Request == null){
	alert('Erro Navegador sem suporte ajax!');
}