Thanks, guys.
DMcG, I will disconnect the Zip and DVD before trying again. I shouldn't think the ram itself is faulty, because I get identical problems when I put the original 256 back in, and the 512 works well in THIS machine. I will try it in another socket. Damage to the motherboard is possible, but I think that I originally tried to install with the 256 strip operating. Can't remember for sure though, I've been through so many permutations in trying to isolate the problem.
JohniK, I've done deep formats(as well as Windows formats),reset the BIOS defaults, and then tried to install direct from legitimate CDs. It gets to the same places and then gives the same problems, although different messages on different OSs. It gets to the same places in 98SE whether I start from floppy or direct from CD.
98 and 98SE get to the point of running windows for the first time, and XP(Pro, by the way) gets to the point of installing the main Windows files.
If there is a virus lurking in there, it is in deep cover. I thought that it was a virus originally, which is why I've done so many formats, but I can't think where it could be. How much software is there in a deep-formatted computer with a reset BIOS? Is there software in the Zip drive? The virus-killer said no HD virus when I started to install originally. I haven't checked the XP CD for virus, come to think of it.
If it is a hardware fault, it seems odd to me why I should get identical problems starting from different places, at different CPU temperatures.
Aren't computers fun!!!!    Chris.