function somenteLetras(evento) { 
    if (navigator.appName == 'Microsoft Internet Explorer') {
		if ( (window.event.keyCode <= 31)||(window.event.keyCode >= 33 && window.event.keyCode <= 64)||(window.event.keycode >= 91 && window.event.keyCode <= 96)||(window.event.keycode >= 123) ) { 
			return false;
		}
    } else { 
		if ( (evento.which <= 31)||(evento.which >= 33 && evento.which <= 64)||(evento.which >= 91 && evento.which <= 96)||(evento.which >= 123) ) { 
			if (evento.which != 0 && evento.which != 8) {
				return false;				
			}
		}
    }
}