Hi Richard,
I think the reinstall (or "install-over") of Win XP Pro is going to be necessary because I'm now detecting other strange behaviour - presumably something corrupted somewhere.
The motherboard is an Asus P3V4X, with the BIOS flashed to level P3V4X105, downloaded from the Asus website on 29/7/2001.
I checked for later updates a while ago and found none.