Hacked Outlook 2003

In my company we use outlook 2003 our mail is stored.Today in my calander that is not shared with anyone I saw an entry on a date I had no meetings the subject was Tuesday and in the notes section someone was typing sentences that made no sense, seemed like code or riddles.

I deleted the appointment and it showed up again under a different date the next Tuesday but no typing this time just the entry.

This is on my internal company network and the internet connection is not firewalled.

Anyone seen this problem before or have any ideas