if(navigator.appName=='Netscape') {
 if(window.innerHeight>500) {
  curhgt=(window.innerHeight-500)/2 }
  else { curhgt=0 } }
else if(document.body.clientHeight>500) {
 curhgt=(document.body.clientHeight-500)/2 }
 else { curhgt=0 }
document.getElementById('mainbox').style.top=curhgt + 'px'