Many thanks for some really useful ideas!

A memory tester has run several clean passes so I tend to think that the RAM is probably OK, although I've currently got it swapped with the good machine to see where the problem next occurs. I could still try it in a different bank, however.

The video card itself has been swapped with the good machine and the bad machine still failed which suggests that the card is OK although I'm currently getting the latest XP drivers for it to see if they help.

I think maybe disabling the auto-restart could prove to be the most informative.

The biggest problem I have is that it could run quite happily for many days before it crashes again!