There is a known incompatiblity between Outlook 2000 and later versions related to reminders on meeting requests sent in iCalendar (aka iCal) format. To resolve this issue, do one of the following:
-- Contact Microsoft Product Support Services to obtain the Outlook 2000 post-SP3 hotfix described in this article: http://support.microsoft.com/?kbid=823199
-- Tell the senders to remove any reminder before sending you an iCal meeting request.
-- Tell the senders to send to you in native Outlook meeting request (RTF) format, not as iCal.

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Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> When I use Outlook2003 and schedule an attendee via the Appointment Tab, the
> attendee gets an unformatted email if they have any Outlook version before
> 2003.
> It comes up as code rather than a user oriented scheduling message.