Before proceeding with this fix, it is highly recommended that you back up the end user's inbox data first.   To do this:

1. Right click on the Desktop
2. Select New > Folder
3. Name it something like "Inbox Backup"
4. Open Outlook Express
5. Left click on the Inbox
6. Left click on the first email on the right hand panel
7. Press CTRL + C
8. Minimise Outlook Express
9. Open the folder you created on the Desktop
10. Press CTRL + V

Once you have done this, you can proceed with the deletion of inbox.dbx.

1. Close Outlook Express
2. Search for inbox.dbx
3. Delete the file
4. Restart Outlook Express

N.B.   I would also check to see if there are any faults with the ISP's POP server beforehand as this can also provoke this error.