allow user to enter only "Numbers" and if required, only one dot "."

Try this:

<asp:TextBox ID="TextBox1" onkeypress="return numericOnly(this);" runat="server"></asp:TextBox>

<script type="text/JavaScript">
<!--
function numericOnly(elementRef)
{
 var keyCodeEntered = (event.which) ? event.which : (window.event.keyCode) ? window.event.keyCode : -1;

 // Un-comment to discover a key that I have forgotten to take into account...
 //alert(keyCodeEntered);

 if ( (keyCodeEntered >= 48) && (keyCodeEntered <= 57) )
 {
  return true;
 }
 // '+' sign...
 else if ( keyCodeEntered == 43 )
 {
  // Allow only 1 plus sign ('+')...
  if ( (elementRef.value) && (elementRef.value.indexOf('+') >= 0) )
   return false;
  else
   return true;
 }
 // '-' sign...
 else if ( keyCodeEntered == 45 )
 {
  // Allow only 1 minus sign ('-')...
  if ( (elementRef.value) && (elementRef.value.indexOf('-') >= 0) )
   return false;
  else
   return true;
 }
 // '.' decimal point...
 else if ( keyCodeEntered == 46 )
 {
  // Allow only 1 decimal point ('.')...
  if ( (elementRef.value) && (elementRef.value.indexOf('.') >= 0) )
   return false;
  else
   return true;
 }

 return false;
}
// -->
</script>


Note that I attached the event handler to the element in the HTML declaration. This will work, but will produce a compiler warning. The proper way to do this for a server control is in the Page_Load handler of the server-side code like this:
 TextBox1.Attributes.Add("onkeypress", "return numericOnly(this);");