var nav4 = window.Event ? true : false;
function acceptNumLetras(evt) 
 {	// NOTE: Backspace = 8, Enter = 13, '0' = 48, '9' = 57, 'A'=65 'Z'=90, 'a'=97 'z'=122 'ñ'=164 'Ñ'=165
    var key = nav4 ? evt.which : evt.keyCode;	
    if (evt.ctrlKey) 	
      return false; 
	else 
       return (key <= 13 || (key >= 45 && key <= 46) || (key >= 48 && key <= 57) || (key >= 65 && key <= 90) || (key >= 97 && key <= 122) || (key == 209) || (key == 241) || (key == 225) || (key == 233) || (key == 237) || (key == 243) || (key == 250) || (key == 193) || (key == 201) || (key == 205) || (key == 211) || (key == 218));
 }