Hola,
Podrias hacer una validacion un poco mas sencilla, algo asi
<SCRIPT language=Javascript>
<!--
function isNumberKey(evt)
{
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return true;
return false;
}
//-->
</SCRIPT>
<asp:TextBox ID="TextBox1" onkeypress="return isNumberKey(event)" runat="server"></asp:TextBox>
Saludos,