<select id="dhtmlgoodies_country" name="dhtmlgoodies_country" onchange="getCityList(this);getValueName(this)">
<option value="">Выберите номер вашего компьютера</option>
<option value="1">Компьютер №1</option>
<option value="2">Компьютер №2</option>
<option value="3">Компьютер №3</option>
</select>

<select id="dhtmlgoodies_city" name="dhtmlgoodies_city">

</select>

<input value="" id="pc_n" name="pc_n"></input>
<input value="" id="pc_p" name="pc_p"></input>
<input value="" id="pc_e" name="pc_e"></input>

var ajax = new Array();

function getCityList(sel)
{
	var countryCode = sel.options[sel.selectedIndex].value;
	document.getElementById('dhtmlgoodies_city').options.length = 0;	// Empty city select box
	if(countryCode.length>0){
		var index = ajax.length;
		ajax[index] = new sack();
		
		ajax[index].requestFile = 'listpc.php?countryCode='+countryCode;	// Specifying which file to get
		ajax[index].onCompletion = function(){ createPc(index) };	// Specify function that will be executed after file has been found
		ajax[index].runAJAX();		// Execute AJAX function
	}
}

function createPc(index)
{
	var obj = document.getElementById('dhtmlgoodies_city');
	eval(ajax[index].response);	// Executing the response from Ajax as Javascript code	
}

// ************************************

function getValueName(inp)
{
	var countryCode = inp.val;
	document.getElementById('pc_n').val.length = 0;	// Empty city select box
	if(countryCode.length>0){
		var index = ajax.length;
		ajax[index] = new sack();
		
		ajax[index].requestFile = 'listpc.php?countryCode='+countryCode;	// Specifying which file to get
		ajax[index].onCompletion = function(){ createPc_n(index) };	// Specify function that will be executed after file has been found
		ajax[index].runAJAX();		// Execute AJAX function
	}
}

function createPc_n(index)
{
	var objv = document.getElementById('pc_n');
	eval(ajax[index].response);	// Executing the response from Ajax as Javascript code	
}

<?php

if(isset($_GET['countryCode'])){
  
  switch($_GET['countryCode']){
    
    case "1":
      echo "obj.options[obj.options.length] = new Option('Москва','1');\n";
      echo "obj.options[obj.options.length] = new Option('Санкт - Петербург','2');\n";
      echo "obj.options[obj.options.length] = new Option('Владивосток','3');\n";
      echo "obj.options[obj.options.length] = new Option('Ростов','4');\n";
      echo "objv.val[objv.val.length] = new value('имя','5');\n";
      echo "objv.val[objv.val.length] = new value('телефон','6');\n";
      echo "objv.val[objv.val.length] = new value('email','7');\n";
      
      break;
    case "2":
      
      echo "obj.options[obj.options.length] = new Option('Киев','11');\n";
      echo "obj.options[obj.options.length] = new Option('Одесса','12');\n";
      echo "obj.options[obj.options.length] = new Option('Львов','13');\n";
      
      break;
    case "3":
      
      echo "obj.options[obj.options.length] = new Option('Нью-Йорк','21');\n";
      echo "obj.options[obj.options.length] = new Option('Вашингтон','22');\n";
      echo "obj.options[obj.options.length] = new Option('Чикаго','23');\n";
      
      break;
  }  
}

?>