Bloquear Letras Campo Text

Função Javascript para bloquear a digitação de Letras em um campo text.

<script language="Javascript" type="text/javascript">

<!--

function blockNumbers(e)

{

var key;
var keychar;
var reg;

if(window.event) {
  // for IE, e.keyCode or window.event.keyCode can be used
  key = e.keyCode;
}
else if(e.which) {
  // netscape
  key = e.which;
}
else {
  // no event, so pass through
  return true;
}

keychar = String.fromCharCode(key);
reg = /[^\d-]/;
return !reg.test(keychar);
}
//-->
</script>
<form>
<input type="text" onkeypress="return blockNumbers(event);" />
</form>