Windows Live won't import all of my Outlook Express 6 Inbox

When I imported my Outlook Express files to Windows Live (I backed them up from my XP machine to an external HD) I got everything except the last 6 months of my Inbox file. Today I went back to the XP machine and created a new folder in OE with just the missing information in it plus one day earlier as a safety margin. When I try to import that folder from my external HD I get the 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 copied the folder to my desktop, tried the import wizard again and got the same message. The file is approx 400MB and is still a dbx file. Is there any way that I can convert it to the file type used in Windows Live other than the import wizard? In order to create the folder I had to open Outlook Express so that I could get just the missing messages I wanted from the Inbox file. If that is the problem, would that OE folder/file be closed if I went back to the XP machine and copied it without having OE running?

Thanks for any help you can give me.

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