This solution works perfectly in all browsers and this is very easy use.
- This is work without Numpad.
onkeydown = "return (!(event.keyCode>=65) && event.keyCode!=32);"
- This is work with Numpad.
onkeydown = "return (!((event.keyCode>=65 && event.keyCode <= 95) || event.keyCode >= 106) && event.keyCode!=32);"