What happens when you try to open it? 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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