Try just replacing the files with the old ones. In windows XP, outlook stores the email files in:
C:\Documents and Settings\{USER NAME}\Local Settings\Application Data\Identities\{BIG LONG NUMBER}\Microsoft\Outlook Express
in 98 I believe it's stored in: C:\Windows\Application Data\Identities\{BIG LONG NUMBER}\Microsoft\Outlook Express
Hope this helps, I copy and replace my email files all the time when installing/reinstalling an OS.