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To restore a bak folder to the message store folder, first locate the Message Store in Windows Explorer.
In OE: Tools | Options | Maintenance | Store Folder will reveal the location of your Outlook Express files. Press the Tab key to highlight the folder location, then Ctrl+C. Close OE, then Start | Run | Ctrl+V will put the location in the box - Click OK and you'll see the OE files. Otherwise, write the location down and navigate to it in Windows Explorer.
In WinXP, the dbx files are by default marked as hidden. To view these files in Windows Explorer, you must enable Show Hidden Files and Folders under Start | Control Panel | Folder Options Icon | View, or in Windows Explorer | Tools | Folder Options | View.
In Windows Explorer, click on the dbx file for the missing, or empty, folder and drag it to the Desktop. It can be deleted later once you have successfully restored the bak file. Minimize the Message Store.
Open OE and, if the folder is missing, create a folder with the *exact* same name as the bak file you want to restore but without the .bak. Eg: If the file is Saved.bak, the new folder should be named Saved. Open the new folder and then close OE. If the folder is there, but just empty, continue on to the next step.
Open the Recycle bin and right click on the bak file for the folder in question and click Restore. Open the Message Store back up and change the file extension from .bak to .dbx. Close the Message Store and open OE. The messages should now be back in the folder.
If the messages are successfully restored, you can go ahead and delete the old dbx file that you moved to the Desktop.