Uable to update public free/busy data

I have an Exchange 2000 server on a Win2k box. Most clients run Outlook 2000 but I have a few Outlook 2002. I got a mixture of them saying they keeping getting the pop-up messing "Unable to update public free/busy data". They click OK and it goes away. Their calendar do get updated. I don't think anybody shares calendar here anyways. Is there anyways to make that message goes away?

Kit