I'm running Windows XP and Internet Explorer browser and Microsoft Outlook 2003.
In some of my Contacts, the person's birthday is recorded.
It therefore appears - usefully - in my Calendar.
However, it seems that it takes effect as a kind of recurring event and thus as each birthday occurs, it crops up more than once.
I can, of course, delete each additional event on an ad hoc basis - but there is presumably a tick box or similar that I can use instead.