Importing folders in Outlook Express

I had to format my hard drive which had Outlook Express 4 on it. Before I formatted it I copied all the idx and mbx files onto another drive. Now that I’ve reinstalled Windows and installed Outlook Express 5, I can’t import the old email folders. I receive an error message: No messages can be found in this folder or another application is running that has the required files open. Please select another folder or try closing applications that may have files open.
I consulted the Microsoft knowledge base and found article Q252603 which deals with the issue. However, the idx and mdx files are not read only and Folders.dbx is present, so their advice didn’t help.
Please advise.
Shulamit