In Win7, navigate to the normal location of the Windows Mail boxes. If you haven’t moved it to a different location, it is the hidden folder C:\Users\username\AppData\Local\Microsoft\Windows Mail\Local Folders then Inbox, Sent Items, etc.
Copy the .eml files (not other files such as winmail.fol) to an external device (or use a network).
Create a folder in Outlook Express (File > New > Folder named, say, Old Inbox), which will initially receive the copied files. Close Outlook Express. Now open both Outlook Express and the folder containing the .eml files and arrange the windows side by side.
Select (highlight) all the .eml files and drag them (strangely, Copy and Paste will not work) to Outlook Express, either to the new folder name in the left pane or to the folder contents in the right pane. Now open Old Inbox and drag the contents to your usual Inbox.