var center = null;

function openWindow(url, naam, breedte, lengte, scrollVar) {
   var settings = "height="+lengte+",innerHeight="+lengte+",width="+breedte+",innerWidth="+breedte;
   if (window.screen) {
   var ah    = screen.availHeight + 2; 
   var aw    = screen.availWidth - 10;
   var xc    = (aw - breedte) / 2;
   var yc    = (ah - lengte) / 2;
   settings += ",left="+xc+",screenX="+xc+",top="+yc+",screenY="+yc+",scrollbars="+scrollVar;
   }
   center    = window.open(url, naam, settings);
}