I get the impression that your old e-mails are not "lost" as in being corrupted. It's just that you can't get at them.
See if you can navigate to the equivalent to this folder:
C: \Documents and Settings\{your main user name}\Local Settings\Application Data\Identities\{7903668F-0537-490A-AB46-D52DE63F83D0}\Microsoft\Outlook Express
Copy the entire contents into the equivalent folder of your new installation and the e-mails should be accessible.
However, this will overwrite any new e-mails you have sent or received since you began using the new installation so you should back these up by simply dragging and dropping them into a temporary folder on your Desktop.
I have done this sort of thing many times when I have rebuilt systems. It's one of the preliminaries that goes with exporting OE Account information, data and, since my most recent cock-up, Sony SonicStage Music databases. But that's another story!
