That's the answer. But at least my Win 98 had the decency to tell me the reason that it had had to restart 10 times because something was changing my C: drive while defrag was running. I did as Spanner says; it was almost certainly McAfee causing the problem. But the Hewlett Packard network "printer disovery" routine would also be a likely culprit. Defrag worked a treat with just Explorer and Systray running.
Incidentally, the purist would say that you should install new programs this way. Trouble is, no Office program seems to want to run in this configuration, so a re-boot is necessary to return the PC to normal.
I wish I knew why "rundll" shows up as running when I do a Ctrl/Alt/Del. I think it's the reason I'm getting a lot of freezes.
There's nothing in the Startup Folder or the Registry to show what starts it.