Appointments cause computer to lock up

Hi,
Hope someone can help. When one of my users sends an
appointment his outlook will take an abnormal (1/2 hour)
to delete the red receipt. It also takes about 15 minutes
to allow the delete key to be pressed. Now for the
strange part. The same thing happens to the uses that get
the appointment. Again taking an abnormal time to
delete. Problem 2 is when a person allows email to be
sent on their behalf and that person receives an
appointment all of the people that can send email for her
also get invited to the meeting. Any help would be
appreciated.

Thanks