<script type="text/javascript">
function pulsar(e) {
  tecla = (document.all) ? e.keyCode : e.which;
  return (tecla != 13);
}
</script>
<!-- -->
<form action="loquesea.asp" onkeypress = "return pulsar(event)">
<input type="text" />
<input type="submit" />
</form>