Thanks Peter.
Disk Utility would get stuck ~ 20% through running it from the hard disk. I booted it from a 10.3.5 CD and ran Disk Repair and it found dozens of "Overlapped Extent Allocations" (or some such) that it claimed to fix. It also claimed to fix the Permissions when I ran Repair Permissions.
However, on rerunning the Disk Repair or Permissions Repair it would claim to fix the same things. ?
Booting from the hard disk and running the Flash installer still gives the same error.
I updated Firefox to 1.5.0.4 and now it occasionally crashes on startup (it just disappears).
The only thing I can think that would cause all of this is that something seems to be mangled on the 30 GB IBM ATA disk. We'll back up what we can and I'll get another disk so that we can replace it and try a fresh install of 10.3.5 or maybe something later.
If you have ideas of other things we can try, I'm all ears. Thanks again.
Cheers,
Scott.
(Who really should break down and get a copy of some disk repair software...)