First make sure that outlook express is not running in the background by going to Task Manager (Ctrl + ALt + Del) , then Processes tab, then select msimn.exe, and click End Process.
Next start Outlook Express then click File > Work Offline
Now try to delete that message from the Draft folder or the Outbox folder using Shift + Del.
Then move or delete all messages from your Inbox, Sent Items, Outbox and Draft folders. (if you have important messages make sure you move them to your own created folder before deleting them)
Delete all messages from the Deleted Items folder.
Click File > Folder > Compact All Folders.
Close Outlook Express, then restart it and click File >Work Online.
That should do it.
Hope this helps. Let me know if you have any questions, and thank you for clicking Accept if you are satisfied with the answer