Use jQuery and the load()
function
Update you links with a class
<div id="menu_lat">
<a class="loader" href="http://localhost:8080/sgf/form_proposta.php">Orçamentos</a>
<a class="loader" href="relatorios.html">Relatórios</a>
<a class="loader" href="proximos.html">Próximos Eventos</a>
<a class="loader" href="chat.html">Chat</a>
</div>
Then in your <script>
tags add this to handle the click event on your links with the class loader.
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.0/jquery.min.js"></script>
<script type="text/javascript">
$(function() {
$('.loader').click(function(event) {
event.preventDefault(); // stop the link loading the URL in href
$('#conteudo').load($(this).attr('href'));
});
});
</script>