Thank you guys for all your input. I've been away for a few days, hence my absence from this thread.
Not sure where to start - it turned out to be a real headache. The first clear thing is I decided I daren't put KDE4 on that couples' computer. I decided against Gnome because although I like it, it too is reputed to be coming up for a major refit in the new year and I didn't want to hit the same problem - sorry Gnome devs if I'm maligning you here
- that of two rather conventional people in late middle-age having to cope with (they wouldn't) the sort of problems KDE4 hit in its earlier days. So I decided on LXDE or XFCE for them.
The headache I had with my DVD problem got me to a point of despair. I could figure out no consistency in what was happening. E.g - I tried re-installing Gnome (this is on my main machine) The medium was checked, checksum done etc. On the first attempt the machine suddenly dropped me to a shell halfway through the install. On the second attempt it did the same part-way through the updating after the install. The third time all went well until I rebooted after the install and it took me to the splash screen - then no further. Total freeze even with Skinny Elephants. Rebooting took me to the same point every time and no further. And by this time, in between all this, I'd run memtest for two hours, I'd run full disk reads on both drives and I'd changed the DVD writer for a new one. The only thing I hadn't checked because I've no diagnostic tool I'm aware of to check it. But I thought that unlikely anyway. The only thing left was the motherboard. I ordered a new one from Ebuyer which turned up the following day. I installed LXDE (I wasn't risking KDE4 again) and was up and running very soon after, no problem.
So what it was about I'm still not sure. Was it the mobo? I'm still not sure - and for a good reason which I'll cover in another thread with a link back to this one. It may be that the two problems are connected.
Anyway, thank you guys again.