screen.height vs. screen.availHeight

[Nairb]

In reading through my lame javascript book I thought "this" was,
fairly reliable cross browser method to refer to available height
(screen minus browser menu, task bar etc.) but these values are both
equal (height only?). Does anyone know what I should use instead?
Thanks

this =
<script language="JavaScript">
document.write('Screen Height is ' +screen.height+ '<br>');
document.write('Available Height is ' +screen.availHeight);
</script>