var newwindow=null;


function showpopup(link,width,height)
 {
   _left = eval((screen.width - width)/2);
   _top = eval((screen.height - height)/2);

   if(newwindow != null)
   {
     newwindow.close();
   }
   
   newwindow = window.open(link, "popup", "left="+_left+",top="+_top+",width="+width+",height="+height); 
   newwindow.focus();
}