<head>

<script language="JavaScript">
<!–
var nav4 = window.Event ? true : false;
function acceptNum(evt){   
// NOTA: Backspace = 8, Enter = 13, ‘0′ = 48, ‘9′ = 57         
var key = nav4 ? evt.which : evt.keyCode;   
return (key <= 13 || (key >= 48 && key <= 57));
}
//–>
</script>

</head>
<body>

<p><input type="text" name="valor" size="12" maxlength="12"
onKeyPress="return acceptNum(event)"></p>

</body>