na = navigator.appName != "Microsoft Internet Explorer"
nu = navigator.userAgent.toLowerCase();
op = (nu.indexOf("opera") != -1);
isIE5 = !na && !op && document.getElementById

function openwin(wid, ht, loc, r) {
 if (isIE5) {
  var w = wid;
  var h = ht
  var lp = (screen.width) ? (screen.width-w)/2 : 0;
  var tp = (screen.height) ? (screen.height-h)/2 : 0;
  var attrs = 'height=' + h + ',width=' + w + ',top=' + tp + ',left=' + lp + ', resizable=' + r + ', toolbar=no';
  window.open(loc, "", attrs)
 }
 else { 
  warnNonIE() 
 }
}  

function warnNonIE() {
 if (!(isIE5)) {
  alert("Bohužel, ale tato funkce je přístupná pouze pro MS Internet Explore 5 a vyšší.")
 }
}