<select name="vyber" id="vyber" onchange="vyberClanek();">
   <option value="0">Vyberte článek</option>
   <option value="test2">Článek jedna</option>
   <option value="test3.html">Článek dvě</option>
</select>

<div id="mistoZobrazeni">
</div>


<script>
function vyberClanek()
{
     var url = document.getElementById("vyber").value;

     if (url != 0)
     {
        if (window.ActiveXObject)
        {
          httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
        }
        else
        {
          httpRequest = new XMLHttpRequest();
        }
        httpRequest.open("GET", url, true);
        httpRequest.onreadystatechange= function () {processRequest(); } ;
        httpRequest.send(null);
      }
      else
      {

        document.getElementById("mistoZobrazeni").innerHTML = "";

      }
}
</script>
<script>
function processRequest()
{
  if (httpRequest.readyState == 4)
  {
    if(httpRequest.status == 200)
    {
      var mistoZobrazeni = document.getElementById("mistoZobrazeni");
      mistoZobrazeni.innerHTML = httpRequest.responseText;
    }
    else
    {
        alert("Chyba pri nacitani stanky
"+ httpRequest.status +":"+ httpRequest.statusText);
    }
  }
}
</script>