Try
here. My first "fix" or "tweak" ever. Made a huge difference to defrag times.
When the homepage opens, click on "troubleshooting" then "windows 98 defrag problems".
The man also helped me with several other problems when I had '98, shame the forums aren't still a going affair.
[Edit] PS, would pay to run a disk cleanup first, if you haven't already.
PSS, all sorts of goodies for '98, here, too. You'll have it running like new in no time. Of course, some of the freeware programs linked to might not be supported any more...