[Ajax] Problemi col POST di un modulo

[cartilio]

salve,
ho qualche problema a far spedire un modulo col metodo POST per una chat Ajax.
Dove sbaglio?

if(!window.XMLHttpRequest){
  var XMLHttpRequest = function(){
    try{ return new ActiveXObject("MSXML3.XMLHTTP")} catch(e) {}
    try{ return new ActiveXObject("MSXML2.XMLHTTP.3.0")} catch(e) {}
    try{ return new ActiveXObject("MSXML2.XMLHTTP")} catch(e) {}
    try{ return new ActiveXObject("Microsoft.XMLHTTP")} catch(e) {}
  }
}

var Http = new XMLHttpRequest();
var wajax = 'http://www.nomesito.com/script/';


function sendChat(){ 
	var param = 'message=' + document.getElementById('txt_message').value+'&userid='+userid+'&chat=2';
	Http.open("POST", wajax+"chat.php?chat="+room, true);

	Http.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
	Http.setRequestHeader("Content-length", param.length);
	Http.setRequestHeader("Connection", "close");	
	
	Http.onreadystatechange = function(){			
		if(Http.readyState == 4 && Http.status == 200){	
			document.getElementById('txt_message').value = ''; 
			document.getElementById('chat-text').innerHTML = Http.responseText; 
		}
	}
	Http.send(param);
}

grazie mille in anticipo a chi vorrà aiutarmi,