Main PC will not boot under windows at all now, but does work under linux - so the problem is Windows, not the computer. It's been getting worse for some time, so my best guess is that something went wrong several updates ago, and has really buggered the Windows setup. I'm transferring work etc. off the PC to my network drive, then I need to think through my options. One is to start from scratch with the existing PC, reinstall stuff I really need, and try to keep to a minimal system. But I suspect that the problem is that XP is now so bloated and slow that it simply won't run reliably on that PC. That would explain why the other XP box also has problems - they're similar specifications, but the other one isn't used much so the only common feature is that they've had the same Windows updates. Linux works for now, but Ubuntu and most others are now so big I wouldn't want to rely on any recent release continuing to work.
Overall I think the best option is to stick to my original plan and buy a good fast modern PC. I think the quad core one I looked at yesterday may be the best bet, but I'm still open to suggestions. I'll try to reach a decision over the weekend, so any suggestions will be gratefully received.