It depends on which version of Outlook Express the old version was. If it
was Outlook Express 4.*, then search the old drive for *.mbx files (which
were the mailbox folders for that version e.g. Inbox.mbx). If the version
was 5.* or later search for *.dbx files. (which are the mailbox folders for
those versions e.g. Inbox.dbx). Once found, look at the properties for
those files (by date and size you should be able to know if they're the
correct files for mail for the identity that you want to import. Take note
of the path of the files.
Then open Outlook Express, click the File menu/Import/Messages, select the
version of Outlook Express you want to Import from (version 5 can be
Imported by selecting version 6 because they both use *.dbx files for their
folders), click the Next button, click to select the option to: Import from
OE 6 Store directory, click the Next button, click the Browse button to give
the path where you found the folders mentioned in the first paragraph. The
rest is self-explanatory.