Hola, cmc.
A ver así:
<form name="form1" method="post" action="buscar.php" onsubmit="return sibuscar()">
<td height="18" align="right" valign="middle">
<input name="Submit" type="button" class="BotMarca"value="Entrar" >
<input name="bus" type="text" class="CajaFormHom">
</td>
</form>
<script>
function sibuscar(){
if (window.document.form1.bus == "")
return false;
</script>
Saludos,
