Have you emptied the Deleted Items folder?

You might need to scan the PST file for corruption. See if this helps: http://www.officeforlawyers.com/outl...ol.htm#SCANPST

Does this happen even if you start Outlook in Safe Mode? (hold down CTRL while you click the icon) See if this helps: http://www.officeforlawyers.com/outlook/tsol.htm#safe