if(document.getElementById) {
  window.alert = function(txt) {
    createCustomAlert(txt);
  }
}

function createCustomAlert(txt) {

  if(document.getElementById("chybovaHlaska")) return;

  pozadi = document.getElementsByTagName("body")[0].appendChild(document.createElement("div"));
  pozadi.id = "pruhlednePozadi";
  pozadi.style.height = document.documentElement.scrollHeight + "px";

  pozadiOkna = document.getElementsByTagName("body")[0].appendChild(document.createElement("div"));
  pozadiOkna.id = "chybovaHlaska";
  pozadiOkna.style.height = document.documentElement.scrollHeight + "px";

  okno = pozadiOkna.appendChild(document.createElement("div"));
  okno.id = "chyboveOkno";
  if(document.all && !window.opera) okno.style.top = document.documentElement.scrollTop + "px";
  okno.style.left = (document.documentElement.scrollWidth - okno.offsetWidth)/2 + "px";
  okno.style.visiblity = "visible";

  h1 = okno.appendChild(document.createElement("h1"));
  h1.appendChild(document.createTextNode("Chyba"));

  msg = okno.appendChild(document.createElement("p"));
  msg.appendChild(document.createTextNode(txt));

  btn = okno.appendChild(document.createElement("a"));
  btn.id = "tlacitko";
  btn.appendChild(document.createTextNode("OK"));
  btn.href = "#";
  btn.focus();
  btn.onclick = function() { removeCustomAlert(); return false; }

  okno.style.display = "block";

  return false;

}

function removeCustomAlert() {
  document.getElementsByTagName("body")[0].removeChild(document.getElementById("chybovaHlaska"));
  document.getElementsByTagName("body")[0].removeChild(document.getElementById("pruhlednePozadi"));
}