I ended up doing an "Upgrade" over the original, rather than a fresh install, as a fresh install was giving problems! Go figure! I still had a few things to sort out, but managed to get everything ok in the end and son is now back at uni with XP working nicely.
Anyway, I have subsequently, while dealing with a completely seperate problem, discovered what I think was the problem with the disc not being recognised as "master", but only as "slave"...
I had bought 3 new HDDs. They are all "Western Digital" OEM, and it was one of these that was giving the problem. Another was to be used in conjunction with an external HDD enclosure, and was bought for a pal. I know, from previous experience, that when using these enclosures the HDD must be set as "master", so I changed the jumper from it's supplied, default, position of "cable select" to "master".
Unfortunately the unit didn't work. The PC wasn't acknowledging that anything had been plugged into the USB socket. I've just returned from the suppliers, who had spent some time on it, including changing the enclosure, (to no avail). Eventually they realised what the problem was...
Apparently for "Western Digital" HDD, the correct jumper setting for it to be "Master" is to have NO jumpers in place. The jumper position marked as "Master" is ONLY applicable if there is also a "slave" drive on the cable!!
This was the situation I was in with my sons PC; When I had it as "Master" it was the only HDD on the cable.
Now the shop has removed the jumper on the HDD that is in the enclosure all is well.
A lesson learned there. There is no indication, (as far as I can see!), on the HDD itself that this is the case. Perhaps with a drive that isn't OEM it's explained in the documentation? Or should I have just known it?
Anyway... all seems to be well now.