I found it. There is a file containing all your removed files. If it gets too big, Outlook Express or XP can't handle it. You can fix it by removing
Deleted Items.dbx
and restarting Outlook Express. The file's location isC:\Documents and Settings\
Username\Local Settings\Application Data\Identities\{
randomstring}\Microsoft\Outlook Express
Posted by Jeff in the comments.
Just note: The Local Settings
and Application Data
folders are hidden by default.