The folders you should have backed up are contained within [DRIVE]:\Documents and Settings\[PROFILE NAME]\Local Settings\Application Data\Identities\[SOME WEIRD NUMBER]\Microsoft\Outlook Express
1.) Setup a mail account in Outlook Express FIRST.
2.) Follow the directory structure of the path above from wherever you have the backup your old partition and copy those files.
3.) Follow the same directory hierarchy given above, only for your current installation of Windows. Paste those files into the folder and you should be set.