function LetraNIF(nNum) {
	var resto,letra;
	var letras = new Array('T','R','W','A','G','M','Y','F','P','D','X','B','N','J','Z','S','Q','V','H','L','C','K','E','T');
	resto = nNum % 23;
	letra = letras[resto];
	document.getElementById('txtLetra').value=letra;
}

function EnciendeBoton() {
  document.getElementById('btValidar').disabled=false;
}
function ApagaABoton() {
  document.getElementById('btValidar').disabled=true;
}
function EnciendeBotonLogin() {
  document.getElementById('cmdLogin').disabled=false;
}
function ApagaABotonLogin() {
  document.getElementById('cmdLogin').disabled=true;
}
<!--
var nav4 = window.Event ? true : false;
function acceptNum(evt){ 
// NOTE: Backspace = 8, Enter = 13, '0' = 48, '9' = 57 
var key = nav4 ? evt.which : evt.keyCode; 
return (key <= 13 || (key >= 48 && key <= 57));
}
function acceptNumReales(evt){ 
// NOTE: Backspace = 8, Enter = 13, '0' = 48, '9' = 57, '.' = 46
var key = nav4 ? evt.which : evt.keyCode; 
return (key <= 13 || (key >= 48 && key <= 57) || key ==46);
}
function acceptText(evt){ 
// NOTE: Backspace = 8, Enter = 13, " = 34, ' = 39 
var key = nav4 ? evt.which : evt.keyCode; 
return (key <= 13 || (!(key == 34 || key == 39)));
}
function acceptTextLoginClave(evt){ 
// NOTE: Backspace = 8, Enter = 13, " = 34, ' = 39, 'barra espaciodora' = 32
var key = nav4 ? evt.which : evt.keyCode; 
return (key <= 13 || (!(key == 34 || key == 39 ||  key == 32)));
}

function validarCorreo(cCorreo) {
  return (/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(cCorreo));
}
function CompruebaCorreo(cCorreo,cNombreTexto) {
  if (cCorreo!="") {
     if (!validarCorreo(cCorreo)) { 
	    alert ('Dirección de correo electrónico no válida'); 			 
		if (cNombreTexto=="email") {
		   document.frmFicha.email.focus(); }
	 }
  }	 
}
function AMayus(cCadena) {
 var cAux=cCadena.value;
 cCadena.value=cAux.toUpperCase();
}