Chat with PHP, MySql, Ajax: How to improve it?

I made this simple chat in php. You have suggestions on how I can improve it?

This file configures the connection to the server

This file is a simple form "login" ( only username required )

 <form action="chat.php" method = "post">
    Username: <input type="text" name="nick"/><br />
    <input type="submit" name="button" value="Go"/>
    </form>
    <?php

    include('connessione.php');
    if(isset($_POST['button'])) {
        $Username = $_POST['nick'];
        $Username = mysql_real_escape_string($Username);
        $Query = "INSERT INTO users (NickName) VALUES ($Username)";
        if(!$Query) {
        }
        else {
            "Error performing query!".mysql_error();
        }
    }
    ?>

This file is the chat!

<script type="text/javascript" src="chat.js"></script>
    <style type="text/css">
    .tastiera {
        width:500px;
        height:50px;
        border-radius:5px;
        border:1px solid #999;
        overflow:auto;
        font-size:13px;

    .chat {
        width:500px;
        height:6000px;
        border-radius:5px;
        border:1px solid #999;
        overflow:auto;
        font-size:13px;
    }

    </style>
    <?php
            include('connessione.php');
            session_start ();
            if($_SESSION['nick'] == “”){
                echo "You are not authorized to enter!";
            }
            exit();
            header('Cache-Control: Private');



    ?>
    <div id="CHAT"></div>
    <form action="salvataggio.php" name="inserimento" method="post" onsubmit=”javascript:location.reload();”>
    <input type="text" name="messaggio" width="500" height="50"/>
    <input type="submit" value="Chat"/>

    </form>
<iframe src="messaggio.php" name="MSG" id="MSG"></iframe>

This Ajax file Update the chat

   function Update()
    {
      return Request();
    }
    window.setInterval("Update()", 3000)
    var XMLHTTP;
    function Request()
    {
      XMLHTTP = GetBrowser(ChangeStatus);
      XMLHTTP.open("GET", "ajax.php", true);
      XMLHTTP.send(null);
    }
    function ChangeStatus()
    {
      if (XMLHTTP.readyState == 4)
      {
        var R = document.getElementById("CHAT");
        R.innerHTML = XMLHTTP.responseText;
      }
    }
    function GetBrowser(FindBrowser)
    {
      if (navigator.userAgent.indexOf("MSIE") != (-1))
      {
        var Class = "Msxml2.XMLHTTP";
        if (navigator.appVersion.indexOf("MSIE 5.5") != (-1));
        {
          Class = "Microsoft.XMLHTTP";
        } 
        try
        {
          ObjXMLHTTP = new ActiveXObject(Class);
          ObjXMLHTTP.onreadystatechange = FindBrowser;
          return ObjXMLHTTP;
        }
        catch(e)
        {
          alert("attenzione: l'ActiveX non sarà eseguito!");
        }
      }
      else if (navigator.userAgent.indexOf("Mozilla") != (-1))
      {
        ObjXMLHTTP = new XMLHttpRequest();
        ObjXMLHTTP.onload = FindBrowser;
        ObjXMLHTTP.onerror = FindBrowser;
        return ObjXMLHTTP;
      }
      else
      {
        alert("L'esempio non funziona con altri browser!");
      }
    }

This file save into database the messages

<?php
@session_start();
if(!isset($_SESSION['nick'])){
  @header('Location:prechat.php');
}else{
  if(isset($_POST['messaggio'])){
    include 'connessione.php';
    $user = $_SESSION['nick'];
    $mex_chat = addslashes($_POST['messaggio']);
    $query = "INSERT INTO utenti (user, mex_chat) 
    VALUES ('$user','$mex_chat')";
    @mysql_query($query)or die (mysql_error());
    @mysql_close();
    @header('Location:chat.php');
  }
}

This file display the messages

$sql = "SELECT user, mex_chat FROM 
utenti ORDER BY id_chat DESC LIMIT 0,10";
$sql_res = @mysql_query($sql)or die (mysql_error());
if(@mysql_num_rows($sql_res)>0)
{
  while ($fetch = @mysql_fetch_array($sql_res))
  { 
    $utente = stripslashes($fetch['user_chat']);
    $mex_utente = stripslashes($fetch['mex_chat']);
    echo '<b>'. $utente .'</b>: '. $mex_utente.'<br />';
  }
}else{
  echo 'Non sono stati ancora inseriti dei messaggi.';
}
?>

What do you think? How can I improve or resolve any bugs ( if there are ) ?