98 computer’s running slower

Hi,
I have a few windows 98 computers. That just seem to be getting slower and slower over time.
I do not install anything. I run a defrag and diskscan constantly, but it does not appear to help. Is there something else I s/be doing to bring back the speed I once had. Do I have to wipe the hard drive and reinstall the os? I don’t think I s/have to.
Any help would be appreciated. Thank You