The problem: After attaching a file to an Outlook email message or saving an attachment, you can't move, delete, or rename the Windows folder the file is stored in and you receive an error message telling you the file is in use.
The action can't be completed because the folder or a file in it is open in another program. Close the folder or file and try again.
This happens because Outlook keeps a lock on the last folder you accessed to Save or Insert attachments. It will release the lock on the current folder when you change folders or restart Outlook.
To release the lock, open a new message form, click Insert file and select a different folder. You don't need to select a file and actually insert it, you just need to select a new folder. Close the Insert file dialog and the new message form. Problem solved. Outlook released the lock on the previous folder and you rename, move, or delete it. It puts the lock on the new folder, so I recommend selecting My Documents or another folder you won't be renaming or deleting.
If you know you will be renaming, moving, or deleting the folder, you can click the Insert button and select a new folder right after attaching the file.