Off topic still, so sorry folks, but *all* my problems were with shop bought machines with Win10 preinstalled.
There was a discussion on the Windows forums about M/S dropping support for lots of hardware. Clobbered one of my machines for playing RPGWorld. 6 months later - no intervening RPGWorld updates and it's working fine.

My Acer laptop died mid update because the M/S techies in their wisdom didn't check available space requirements for an update and at that time update would only use the C drive. Thankfully an M/S support exec was able to reinstate things.

It was a different story for my MSI laptop which completely lost it's O/S with no way to recover without buying another copy of Win10.

My MSI desktop was turned into a very expensive brick and sits there , in the corner of the office to this day. I *will* not give M/S another red cent.

So yes - if I could identify the staff involved I would like to reconfigure them. Hate? Oh yes.

But I now have a set of working machines - all Win10 - and I like it - nearly as much as I like XP

No idea how that could have happened unless you had catastrophic hardware failure, Win10 doesn't use keys like other versions of windows, it snapshots your hardware, then when you install it again it checks that to grant you access to windows, only way i can think of that not working any more is if your bios has had a major failure and wiped itself.

Win10 is storing the key in the uefi bios.

Thanks for the heads up @synchromesh, have found and downloaded. Excellent!

Anyone else - I know this is neither a tech support nor XP forum, but just in case a knowledgeable soul is around - I just tried to Ghost my XP drive and Ghost, having stuck a new partition in the middle of my main partition, installed PC Dos into it, set it's new partition as bootable, then gave up. I'm left with a PC that boots to PC Dos, and despite offering, won't revert to Windows. Am I right in thinking if I delete the PC Dos partition and set the original partition as bootable I'll be okay?

Did you make a partition first or did ghost make it? I always used a second drive for it not sure what use a ghosted image would be on the same drive. Also makes it easier to get back to windows if it fails. If you can make the main partition bootable, I'd do that first then delete the partition from windows.

Now that sounds like sound advice DVader - so long as Windows doesn't trip over the overlaid partition.

No - I didn't make the partition. I cannot think how I could. Certainly not through any application I have.

I'll give it a go. It surely won't make anything worse.

Disk Management has all the options to create and manage partitions Just be careful you choose the right one. Used to build and repair PC's as part of my job, so used Ghost a fair bit, been awhile now though.

as dvader says, you don't need any special software, windows can do it for you - just type disk manager into your start bar and it'll bring up the option

I built my first PC in 1988. I built my last one in 2002. It's been a long time since I p* about with such low level stuff, much has changed and confidence has waned.
Would I trust Windows with low level disk management after the s* Microsoft have dealt me in the last few years? Absolutely. Not.
Testdisk I trust.
If I don't post again then the worst has happened and I have lost a long-time friend. Sob. Sob.

If you have your windows disk you may be able to repair it from that as well.

I've used windows to partition drives for years, never had an issue btw. I wouldn't worry about using it myself.

Success. Happy boy.
Now if I can just get my dead Win7 and Win98 desktop's going again. . .