Humm. Your Google search results different than mine.
The very first hit is here
http://www.olympus.net/olympusnet/olympusonly/spam/Outlook/spamFilterOutlook.html
entitled "CREATING A SPAM FILTER IN OUTLOOK 98 OR 2000"
Download.com, again from the Google Search shows
http://download.com.com/3150-2382-0.html?tag=dir which includes Outlook
98 in the title.
Again, I can't vouch for any of these.
Outlook 98 getting very old. My hunch is that most developers won't
bother making/supporting anything for it. It was developed in c. 1995
through c. 1997 before spam was the problem it is today.
Look to see if your mail service provider (ISP or corporate) has spam
filtering available. Probably better than anything you can add to
Outlook (even the newer versions).
Hope this is useful to you. Let us know.
rms