Sorry to say it, but what is lost is lost. In the future you can go to this location:
X:\Documents and Settings\(User Name)\Local Settings\Application Data\Identities\{4EE96ED2-7E97-47D9-9851-E3D38D882E37}\Microsoft\Outlook Express
X being the drive letter of your XP installation and (User Name) being your user name.
Copy the entire folder "Outlook Express" to a safe location, preferrably another partition or drive, before you do a reinstall and then when you're back up and running you can open Outlook Express, Go to File> Import> Messages... and choose "Microsoft Outlook Express 6 (or earlier versions if applicable) and it will allow you to restore all your messages from the directory you saved (the outlook express folder you saved).
That's what i do when i'm "planning" a clean install. I know that doesn't help with what you've already lost, but i hope it may help you next time.