OK, a progress report on the freshly bricked PassingCloud PC (PCPC?) and a request for help, if anyone at Avast HQ is reading this thread. (And if so, I hope this business is giving somebody a few grey hairs.)
Running a registry repair utility (ReHive) from a boot disc didn't solve it. The program corrected what it could in the files it found, but could not make the machine run again (it still generated BSODs after rebooting). Fortunately, this time (unlike when some rogue software decided to override my preferences and update itself) I had an image of the prior setup, so restored it.
The first Repair Install, having found the assorted CDs etc. that I thought I wouldn't be needing again, did not work either. I've never before seen Setup reboot into Safe Mode before declaring that it cannot run in Safe Mode, but I have now. Restore. This morning, I have been running another Repair Install, which again rebooted into Safe Mode, declared it couldn't run in Safe Mode, and re-rebooted again. Back to the Boot Disc, and I removed the switch in boot.ini responsible for switching to Safe Mode, and rebooted again. Improvement: Setup continued, switched to graphics mode and continued setting up. Hope!
For a few minutes. Halfway through the "Installing Windows" phase (it says another 34 minutes, but the thermometer graph is at about 2/3 complete) Avast strikes again. Up pops a window titled "AVAST Software Uninstall Utility", telling me that it wants to run in Safe Mode and asking "Do I want to reboot NOW into Safe Mode and run it from there?" or "Do I want to run it from here, not in Safe Mode, anyway?". It has somehow interfered with the Setup process which was looking good, and again is not giving me the option I need (i.e. "No, go away, I'll run you later"). The Setup process continues behind it, but still fails to boot properly - now, whenever it starts, it just reboots quietly (no BSOD).
No. I do not want to run the Avast Uninstaller AT ALL at this stage. I want to get my system working again, then I'll run the Avast uninstaller from Safe Mode later, when I can actually reach Safe Mode again. So, if anyone at Avast can tell me from where in the startup process, exactly, this uninstall utility is being started, so that I can get rid of that call and actually finish setting up my Repair Install before continuing with the Uninstallation from Hell, I'd be grateful. It would be helpful to know where the actual file is, too, so I can run it later. I assume this isn't the uninstaller I downloaded from a link on this forum, which I had intended originally to run when Phase 1 of the uninstallation process unexpectedly crashed my system in flames.
BTW, the posters here banging on about "obsolete operating systems" and "several versions out-of-date software" are missing the essential point. My "obsolete" machine, with its "archaeologically interesting" OS and software, was carefully set up by someone who knew what he was doing (me) and driven by the same person. It has been online all day, every day for years, and has not succumbed to anything unexpected in all that time. IF Avast had asked me to update, as it should have, that knowledgable operator could have imaged the whole thing beforehand, as I do for all other upgrades, and all these headaches would have been avoided as I worked out what was broken and how to fix it. I have no problems with Avast AV's competence (it's been excellent), but no way am I letting it back in if it's going to disregard my settings.