USUAL DISCLAIMER: These instructions assume you are familiar with the Windows Registry. You follow these instructions at your own risk. I cannot be held responsible for anything arising from these instructions. Make a backup first. NO WARRANTY.
This assumes that you've kept the email store in the same folder as before the upgrade - if you've moved it, these instructions will not work (I keep my email store on a different drive, and set things up so that drive gets the same letter after the install) I also keep my NTUSER.DAT and copy from that after the upgrade.
On the old computer/install, export the following keys: HKEY_CURRENT_USER\Identities and HKE_CURRENT_USER\Software\Microsoft\Inte
Alternatively, if you're copying from a saved NTUSER.DAT, load that in regedit. Export the Identities and Software\Microsoft\Internet Accounts Manager keys. Open the .reg files in a text editor, and patch the key names to match up with those above.
On the new computer/install, run Outlook Express once to create initial registry settings, then close it. Open regedit. Delete the subkeys under HKEY_CURRENT_USER\Identities, then merge in the two files from before. Reopen Outlook Express, and you should see all the email accounts and saved emails you had before the upgrade. You'll need to re-enter any saved passwords for the accounts.