I can't believe it, my problem is fixed. I started from scratch, re-partitioned,formatted and installed XP, I carefully installed the nVidia MOBO drivers, then the ATI drivers and rebooted. After the XP logo appeared the monitor went black and cut off. It had been suggested to me that the monitors refresh rate might not be compatible with the ATI graphics card, something that Besmirch had touch on in an earlier post and which I was too slow to pick up on.
So I replaced the spare monitor I was using with the monitor I intended to use when /If, my system ever got up and running. I rebooted and BINGO! I was presented with a nice new XP desktop. It's been a steep learning curve for me and I don't know if my nerves would have taken it if changing the monitor hadn't worked, but there we are. Many thanks to all who have chipped in with advice.