If you get this error in Outlook Express, it means that some of your .dbx files (the folders) have been corrupted. Why? If you have deleted a lot of e-mails or you have deleted the %temp% folder from Windows, these actions could have caused the corruption.
The best thing to do is to restore your backup if hopefully you have one.
Just delete all the .dbx files and copy paste the ones from the backup.
If you don’t have a backup, you can just recreate the inbox folder. Check out this tutorial here that will show you how to recreate / move a folder in Outlook Express.
But as I said, best thing to do is to delete all the dbx files and restore them from a backup.
If nothing works, reinstall Windows. Keep your dbx files copied somewhere on an external hard drive and put them back into the dbx folder after you reinstall Windows.