TextBox accepts only numbers

<asp:TextBox " runat="server"        id="txt1"        onkeypress="return isNumberKey(event)" />

This is the javascript method that will be called by the textbox on onkeyPress event
<SCRIPT type="text/javascript" language=Javascript>

<!--  
  function isNumberKey(evt)    
 {
        var charCode = (evt.which) ? evt.which : event.keyCode
        if (charCode > 31 && (charCode < 48 || charCode > 57))
            return false;
         return true;  
   } 
    //-->
</SCRIPT>