Question about Outlook Express

I'm having a continual problem with Outlook locking up and displaying the spinning beach ball.  This will occur at random 10-20 times a day, and will lock Outlook up for anywhere from 15 seconds to 5 minutes.  The system starts frantic disk activity at the same time- activity monitor shows disk write traffic maxing out the drive, and the system itself often becomes somewhat unresponsive for the entire time.

My Inbox has only about 40 messages in it and I have rebuilt the database.