I will ask for forgiveness instead of Egghead. He was probably kind of sleepy when he posted and gave you the default location for Outlook instead of Outlook Express.
To know the location is actually quite easy. Inside Outlook Express go to Tools and open Options.
There go to the Maintenance tab. There is a button in the middle f the windows that will say Store Folder. That is the folder that you want. Click the button and a small window with the location will appear. Select the whole location (Which will be something like windir%\Document and Settings\Username\Local Settings\Application Data\Identities\{2985-h2408-d39085-49e098-90f854}\Microsoft\Outlook Express\)
Have in mind that the winddir% is replaced by the drive where your Windows is located and the Windows folder (usually Windows, or Winnt) After the Identity also, is it going to be a unique number. In Windows 98, all profiles will be inside the identities folder, but with different number since they are unique.
OK, once you located the folder, copy the location and paste it in the address bar of windows explorer (or Internet Explorer will do too)
That should open the location, where you are going to have a file for each folder that appears in Outlook Express. If it doesn't appear, remember that Local Settings folder is a hidden folder, so you have to enable "Show hidden files and Folders" from the Folder options menu of an Windows Explorer window.
Now, I guess that you might want to know also how to move it to the other computer, right? It is not as easy as to just put the folder in the new computer.
First you will have to create the account in the new computer. You can do this a lot faster if you export the account settings (from tools>Accounts) and then import them back into the new computer (or destination computer)
Once you did that, you will have to locate the store folder in the destination computer. Remember that I say that identities IDs are unique? Well, Outlook Express created another one, which is different even if the settings are the same. Once you located the folder (same rules as before, just that I think you mention Windows 98, which doesn't have Document and Settings. It should be inside the profiles or Users folder. I can't remember now) open the folder that Outlook created and copy the files from the other computer there.
Please make sure you haven't actually downloaded new email with the new account because you will overwrite them.
If you already have email, that is OK, just go to File>Import, this will open the Outlook Express Import wizard. Depending on the Outlook version that you have in the destination folder you have to chose Microsoft Outlook Express 6 or 5, then click next. In this part select the second radio button, which says "Import email from OE6 Store directory" point to the location where you copied the files and from there it will ask you where you want the mails to be stored. Default selections should be fine from here.
Quick note, I had noticed when using Multiple identities in Outlook Express under Windows 98 that sometimes the address book get mixed. Just to let you know