Defrag locks-up in Win98

I have an installation where the PC locks up everytime Win98’s Defrag is run. Diskeeper trial version works fine. I have reloaded the Win98 O/S and the problem has returned. I can’t find a way to reload only the defrag application and I have not found any other reports of this failure in any of the technical web sites. I hope that someone here can help shed some light on this issue. Your help is greatly appreciated