Start-> Run; outlook.exe /cleanreminders
If that doesn't work, use scanpst.exe to check your pst-files for errors.
See http://www.msoutlook.info/question/77

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Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
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http://www.msoutlook.info/
Real World Questions, Real World Answers
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> Every time I open Outlook I keep getting reminders for long past calendar
> or
> tasks items. I get the notice 'cannot turn off ... one or more parameter
> values are not valid.' Sometimes I have to close Outlook and open again,
> when
> of course the same reminders and message comes up again. What can I do?