Change default calendar form

Outlook 2007. I can create a form with a selected category and publish it.
I can right click my calendar, select the form from "When posting to this
folder use" in preferences and successfully create a new calendar event and
it has the new form with the category slected.

My problem is that if I close Outlook and then reopen it, the preference has
gone back to IPM.appointment and I no longer have the category defaulted. I
must change the preference every time I open Outlook.

Is there a way to keep the preferred form or some other way to automatically
create appointments with my selected default category.

I'd hate to have to do it manually every time I open Outlook.

Thanks.