outlook express

My guess: a corrupted .dbx file.

I'd start with making a copy of the folder with all that dbx files to a very safe place. Then delete all contents and fill it from your backup. Now there are 2 possibilities:
1. It still doesn't work. Then it's another issue, so you can copy the old situation back after you solved that other issue.
2. It works. Then gradually (one by one) copy the dbx-files (not the folders.dbx, but the ones with mails in it) from the new copy (the most current one) over the old one from the backup until you encounter the corrupted one. Then you lost the contents of that folder that you added after making that last backup, but that should be all.

Kees

From your desktop go to Explorer and find your OE store folder and delete only the Folders.dbx file. Afterwards, when you open OE this file is recreated so if it is now corrupt the problem will be fixed. If it is some other problem the suggestion by Kees_B should fix it.