OK guys, thanks to the leads here I have done it!
To start with, this
MSKB article offers four possible methods. 1 2 3 are not usable with a dead computer but #4 "should be". You need a real XP boot CD and then use the recovery console to uninstall SP3.
Except that this didn't work.... SP3 stayed installed, or at least partially. And the machine remained dead. One could use F8 to enter the Safe Mode and this started up OK, for about 5 secs, and then another BSOD with the same 'corrupted registry' message...
Method #4 ran as described in the MSKB article (with loads of error messages) but evidently failed because most of the files it was trying to restore were 'access denied' - the usual windoze issue.
M$ offer a free "SP3 disaster support line" in the USA (as mentioned in the blog referred to above) but on that US number they won't discuss it with anybody outside the USA. They gave me a UK # but they don't have the SP3 support option. However, if you call it - 0870 6010100 - and select the option for the pukka business customers with prepaid support contracts (the £200/phone call kind of thing) and fairly decisively state that you have a dead ex-SP3 machine and that M$ claims to offer free support on SP3 disasters, they will support you and assign a case number to you and you get transferred to a "specialist".
So... I spent about 2 hours on the phone to a chap in India. Actually he wasn't too bad. He was very pleasant but didn't know anything I did not know already or did not find in 5 mins on google but he clearly had staff behind him who knew of the many ways SP3 can kill machines and he was frequently referring to this 2nd level support.
He went straight to the above mentioned MSKB option #4 but that didn't work even 2nd time. Eventually he suggested doing a
Repair - this needs the XP boot CD too and you press R at the installation menu. Like most laptops, the Thinkpad did not come with an XP CD but I pulled one out of an old XP SP2 developer's kit
The Repair process claims to take ~ 35 mins but actually it took more like 10 hours. The machine looks like it has crashed and then wakes up after a while. It asked for some files I didn't have but luckily I had them saved:
I had earlier managed to grab all the data off the HD by entering the Thinkpad Diagnostics (located in the ROM) and these have enough network functionality to enable the HD, or specified files, to be copied to a networked computer. I thus saved all the data to another PC - all 20GB of it! This data set could not be used as a whole because it had the duff SP3 installation, but obviously one could pick off selected files from it, as well as any normal application data.
I used this data set to locate the files which the Repair process was after. They were on the HD all along but windoze was too stupid to find them, and the Search function (right click on a folder etc) is not functional during the Repair process so you cannot use it to search the HD of the machine that's being repaired
The Repair process also asks for the XP license key, which luckily was stuck on the bottom of the laptop....
Now it is back up and running. A few quirks with a few things and XP says it is not activated and I have 30 days to do that...
And yes the HD is a SATA one which without a doubt is why Trueimage would not find the HD. But, in the BIOS, there is a choice of two modes, AHCI and Compatibility. I used the latter during the Repair process and then set it back to AHCI. Unfortunately this stopped the machine booting!! Obviously this is the old windoze story where SATA appears as SCSI and one has to insert a floppy (!!) disk with the SCSI driver while pressing F6 during boot-up. I will have to deal with this separately; I do have a USB floppy drive. Windoze did discover a new "RAID" device during a subsequent startup but failed to find the drivers for it... so I am back in the Compatibility mode on the HD which runs just fine. I have no idea what the SATA driver file is called but will look for it another day. It's just as well that the Thinkpad BIOS has a mode where the SATA hardware can be made to look like bog standard IDE!! But it's a useful lesson learnt - Trueimage won't restore to a SATA drive unless you get a recent version so many of my "image backups" (done off desktop machines with SATA controllers) are probably useless, especially as there are loads of SATA controllers out there, and most of them don't have the IDE lookalike mode. However, TI did not work even in Compatibility mode, but worked after a reinstall.
A few progs had to be reinstalled. Daemon Tools (for mounting ISO images as virtual drives) needed a reinstall. Acrobad writer v7 did its own reinstall (?). So far, it looks like a success. And the Windows Installer Service needs to be updated, according to Acrobat 7 which tries to reinstall itself on every startup.
The RAID driver (which got trashed by the Repair procedure) is
here and yes it needs to be copied to a 3.5" diskette...
Automatic updates are of course
disabled now
A friend works for a huge (vast) IT hardware company and they won't touch SP3 with a bargepole.
THANK YOU ALL.