var pop = null;

function popdown()
{
  if (pop && !pop.closed) pop.close();
}

function popup(obj,w,h,s)
{
  l=(screen.width)?(screen.width-w)/2:100;
  t=(screen.height)?(screen.height-h)/2:100;

  var url = (obj.getAttribute) ? obj.getAttribute('href') : obj.href;
  if (!url) return true;

  if (s>0)
  {
    var args = 'width='+w+',height='+h+',top='+t+',left='+l+',scrollbars=yes,resizable';
  }
  else
  {
    var args = 'width='+w+',height='+h+',top='+t+',left='+l+',resizable';
  }

  popdown();
  pop = window.open(url,'',args);
  return (pop) ? false : true;
}