Outlook 2007's been giving me headaches since I got my new Dell laptop. Every time I edit an existing appointment, the Recurring option is set in my calendar (and my calendar only) without me ever seeing or opening the Recurrence dialog. Major hassle, since I then need to delete the recurrence, which sends an update to all my invitees, who were not even aware of the issue, and who get to accept the same request for the 2nd time. This would also happen, by the way, when I dismiss a notification alert.

Today I found a thread on the Microsoft forums, stating it's probably a Dell issue, related to some improper or leftover software installation they ship their laptops with. Anyways: removing the 'Outlookaddinsetup' from the control panel solved the issue. I cannot even imagine what purpose the program should serve in the first place, but I'm sure glad the issue is gone.