Can't save appointments in Outlook 2K

All of a sudden, my calendar won't let me save appointments. I get a
message that says, "Reminder will not appear because the item is not in your
Calendar or Task Folder. Is this OK?".

If I click "Yes" the appointment screen closes, but the item is not in my
calendar. If I click "No", the screen does not close.

I recently installed ACT! 2007, which uses Outlook for email. I suppose I
could uninstall ACT! and see if this solves the problem. I did a hard
reboot, but that didn't help.

Any ideas?

Thanks in advance,

Robbie

PS. ACT! 2007 is very unstable. If you are considering buying or upgrading
to ACT! 2007, I would consider other options.