Scandskw & Defrag freeze Win 98SE

I have Scandskw & Defrag as Sheduled tasks in Win 98SE and recently each has frozen Windows. I tried each one manually and Windows froze (except the mouse cursor). I’ve used System File Checker – says all OK, and I’ve re-installed Windows (without deleting anything) – no change.
(I don’t know of any installation, etc that could have caused this.)
Any answers?