Put any mail from the Outbox and Sent Items folders that you want to save into another folder,then close Outlook Express.

Go to Start > Search > For Files and Folders.
Click on "All files and folders" and from the "More Advanced Options" drop down menu put a check in Search hidden files and folders

Now search for Outbox.dbx and Sent Items.dbx and delete both of these files.
Restart OE and hopefully the problem is now fixed.