Go into Outlook Express, click tools/options and select the maintenance tab. There is a button called store folder, click it, and that will tell you where Outlook stores your email. Set both the Win2k and Win98 versions of Outlook to point to the same directory/same drive - eg. c:\program files\email\, and then if you are in either win2k or win98 you will get the same email in your inbox.
Make sure it's the same in both win2k and win98. Also it only works with Outlook 5 and above so you may have to upgrade in win98(or set win2k version of outlook to point to win98 Outlook message store)
Hope it helps.