listo
<html>
<head>
<script language="JavaScript">
<!--
var nav4 = window.Event ? true : false;
function acceptNum(evt){
// NOTE: Backspace = 8, Enter = 13, '0' = 48, '9' = 57
var key = nav4 ? evt.which : evt.keyCode;
return (key == 95 || (key >= 48 && key <= 57) || (key >= 64 && key <= 90)|| (key >= 97 && key <= 121));
}
//-->
</script>
</head>
<body>
<p><input type="text" name="valor" size="12" maxlength="12"
onKeyPress="return acceptNum(event)"></p>
</body>
</html>