I solved this one myself. I must be getting better at this.
Just to recap, I wanted to import my email files from a previous installation of Outlook Express, following a computer motherboard failure necessitating a complete rebuild of my hard drive.
With a heavy heart, I tried to access the Win XP help pages at Microsoft.com. I say with "a heavy heart" because I have always found any 'help' on that site to be a) almost impossible to find; b) unintelligible gobbledegook when I've found it and c) it's not for my version of Win XP/Outlook/Internet Explorer anyway, etc. etc. It's really just an online version of the equally obscure, thick manuals which used to accompany their software, and which I only ever used as doorstops.
Well, I couldn't even find the Win XP site to begin with, which shows you just what an expert at computing I am. However, in my aimless wanderings around the web, I chanced across this page:-
http://www.insideoe.com/faqs/how.htm#importdbx
which included full step by step instruction written in plain layman's terms, of how to achieve exactly what I was looking for.
I was so surprised I thought it must be too good to be true, but nay .... when I looked further it all looked convincing. So, with a rapidly lightening heart, I took the plunge and followed the directions to the letter.
Result! Now my new installation of Outlook Express 6 has imported all the files from the old installation, allowing me access to years of my previous emails both received and sent, many of which include valuable release codes for software bought on line. In decades of computing involving many upgrades to newer PCs, this is the first time I have been able to salvage the emails from the earlier machine.
Moral of the story: look around and you may find better 'help' for your software than on the manufacturer's own site - certainly Microsoft's!