You may want to try this:

Symptom:
One or more old reminders are generated each time Outlook is started.
If you try to "Dismiss" the reminder then error message appears saying:
Cannot turn off the reminder. You may be reminded again.
If you try to "Open" the reminder then you get the error:
Cannot open the item for this reminder.

Possible Cause:
This error is probably indicative of some corruption within Outlook. It has been observed after an Exchange server has crashed and needed to be rebuilt.

Remedy:
Shutdown Outlook. Restart Outlook with the '/cleanreminders' switch, the easiest way to do this is Start > Run and in the Run box enter:
outlook /cleanreminders

If any reminders still occur then try dismissing or opening them in the conventional way.

Got this from >> http://www.cryer.co.uk/brian/windows...k_cnotiftr.htm