How to subbmit part of the form, without refreshing page

Code:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
   <title></title>
     <script type="text/javascript">     
       function getRequestObject() {
         var req = null;
         if (typeof XMLHttpRequest != "undefined")
           req = new XMLHttpRequest();
         if (!req && typeof ActiveXObject != "undefined") {
           try {
             req=new ActiveXObject("Msxml2.XMLHTTP");
           }
           catch (e) {
             try {
               req=new ActiveXObject("Microsoft.XMLHTTP");
             }
             catch (e2) {
               try {
                 req=new ActiveXObject("Msxml2.XMLHTTP.4.0");
               }
               catch (e3) {
                 req=null;
               }
             }
           }
         }
         if(!req && window.createRequest)
           req = window.createRequest();
         return req;
      }

      function processRequest() {
        var req;
        var p = document.form1.text1.value;
        uri = 'ajax.php?test=' + encodeURIComponent(p);
        req = getRequestObject();
        req.open('GET', uri, true);
        req.onreadystatechange = function() {
          if (req.readyState != 4) return;
          if (req.status==200) {
            if (req.responseText) {
              processResponse(req.responseText);
            }
          }
          delete req;
        }
        req.send(null);
        delete req;
        return true;
      }
      function processResponse(text) {
        document.getElementById('status').innerHTML = text;
      }
    </script>
  </head>
  <body>
    <form name="form1" method="post" action="submit.php">
      <fieldset>
        <p>
          <label for="">Text 1</label>
          <input type="text" name="text1" value=""  />
          <input type="button" value="ajax get" onclick="processRequest();" />
        </p>
        <p>
          <input type="submit" name="submit" value="submit" />
        </p>
      </fieldset>
    </form>
    <div id="status"></div>
  </body>
</html>

Code:
<?php
if(!empty($_GET['test'])  AND trim($_GET['test']) != '')
  echo 'Ovo je poslato PHP-u: <strong>' . $_GET['test'] . '</strong>';
else
  echo 'Greska: Nista nije poslato. <br />Potrebno je uneti podatke u polje "Text 1"';
?>