var windowObjectReference = null; // global variable   
  
function openPopupWindow()   
{   
  if(windowObjectReference == null || windowObjectReference.closed)   
  /* if the pointer to the window object in memory does not exist  
     or if such pointer exists but the window was closed */  
  
  {   
    windowObjectReference = window.open("systemrequirements.html", "systemrequirements", "resizable=no,scrollbars=no,status=no,width=412,height=270");   
    /* then create it. The new window will be created and  
       will be brought on top of any other window. */  
  }   
  else  
  {   
    windowObjectReference.focus();   
    /* else the window reference must exist and the window  
       is not closed; therefore, we can bring it back on top of any other  
       window with the focus() method. There would be no need to re-create  
       the window or to reload the referenced resource. */  
  };   
}