Try this from Microsoft :
To resolve this behavior, move all of the .dbx files to another location, and then import them back into Outlook Express:
Create a folder named BackupMail on your Desktop.
Click Start, point to Find, and then click Files or Folders.
In the Named box, type *.dbx, and then click Find Now. Note the path of the file name and location of your mail folders (the files that end in *.dbx).
Note that if you use identities, users all have their own mail folders.
Copy and paste the .dbx files in the mail folders into the BackupMail folder that you created on your Desktop. Verify that the Folders.dbx file has been copied to the BackupMail folder.
This file is necessary for the import functions in Outlook Express.
Start Outlook Express 5.
On the File menu, point to Import, and then click Messages. Click Microsoft Outlook Express 5.Click Import mail from OE5 store directory, and then locate the mail folder that you created.
When you receive the prompt to select the mail folder that you want to import, click All Folders.