function somenteNumeros(evento) { 
    if (navigator.appName == 'Microsoft Internet Explorer') {
		if ( (window.event.keyCode<48)||(window.event.keyCode>57) ) { 
			return false;
		}
    } else { 
		if ( (evento.which<48)||(evento.which>57) ) { 
			if (evento.which != 0 && evento.which != 8) {
				return false;				
			}
		}
    }
}