function popupCentered(url, title, width, height)
{
  var left = screen.availWidth/2 - width/2;
  var top = screen.availHeight/2 - height/2;
  var wf = 'scrollbars=yes,menubar=no,toolbar=no,width='+width+',height='+height+',left='+left+',top='+top;
  window.open(url, title, wf);
}

function isItemSelected(obj, msg, msg2, lLimit)
{
  if( parseInt(document.getElementById('sum_without_pvm').value) < lLimit )
  {
    alert(msg2);
    return false;
  }
  
  var elem = obj.elements;

  for(var i=0; i<elem.length; i++)
  {
    if( elem[i].type == 'checkbox' && elem[i].name != 'pre_billing' )
      if( elem[i].checked )
        return true;
  }

  alert(msg);
  return false;
}