function centra(url, width, height) 
   { 
   if (document.all) 
      { 
      var x = window.screenLeft; 
      var y = window.screenTop; 
      var w = window.document.body.offsetWidth; 
      var h = window.document.body.offsetHeight; 
      } 
  else 
   { 
   var x = window.screenX; 
   var y = window.screenY; 
   var w = window.outerWidth; 
   var h = window.outerHeight; 
   } 
  var cntx = x + Math.round((w - width) / 2); 
  var cnty = y + Math.round((h - height) / 2); 
 window.open (url, null, 'left=' + cntx + ',top=' + cnty + ',width=' + 
width + ',height=' + height);  
}