var popWin = null;    
function openurl(url)
 { 
   closePopWin()           // close any previously opened pop-up window
   popWin=window.open(url,'NewWindow','width=585,height=465,menubar=no,toolbar=no,location=no,scrollbars=0,top=225,left=425');
} 
function closePopWin()     // close window if there is one opened already
{   
   if(popWin != null) if(!popWin.closed) popWin.close() 
}