Just an update, so you don't think I'm ignoring you.
The various previous ideas never did work, though the really irritating bit is that I still can't disable auto re-start after the BSOD, so I had to resort to getting my DSLR out and learn how to shoot video
Turned out to be a Stop 0x0000007F error, which I couldn't get BlueScreenView to record as the PC was failing to write the minidump file.
I has an inspired thought and wondered if Avast was blocking it. Sure enough, when I uninstalled Awast, Minidumps started to be written, pointing to nvgts.sys NVidia's SATA Performance Driver.
This seems to be something to do with the disk management, well after the RAID drivers have been invoked, and during the XP boot process.
I don't think the driver is corrupted, but I guess it must be conflicting with something which was modified in Windows Update. I managed to uninstall one of the updates, two didn't install anyway, so I couldn't remove them and the other says that you can't uninstall it.
Now in the process of backing up the faulty C partition so I can still at least get back to this point when I start delving deeper.
Also annoyingly I can't get SFC /scannow to work from within Safe Mode, doubly annoying considering how long it took me to make an X-SP3 slipstreamed disk for it to verify against.
Does anyone know a cure for that one please? ( I understand it doesn't work fom withing XP Recovery Console either)
I'll see if I can boot to BART PC and try and run it on a remote Windows installation from there.
I'd wlcome any further inspiration for when this backup finishes in an hour or so.