Thanks again - I've been through the list as far as the Mr Fixit tool (which isn't designed to work in XP), but to no avail.
One factor which may be relevant is that the problem definitely isn't with either of the CD/DVD ROM drives itself or its connections, because if I move the EIDE CDROM to Master it works fine (but I lose whatever is attached to slave), and similarly, if I connect my SATA CDROM to any SATA port other than #2 that too works fine.
So the problem seems to lie with whatever part of Windows recognises these two ports, rather than with the attached devices themselves.
Last week I experienced a new symptom: The screen froze, with message "can't play video - change setting to 1680 x 1050@ 60 Hertz" After the necessary reboot I checked and the screen resolution is already at that setting. I therefore used Drivermax to find new drivers and installed any suggested, including Intel Q35 Embedded Graphics Chipset: 6.14.10.5218 2010-01-13
This may have been a mistake! I'm now getting the blue screen of death 3 or 4 times a day with the message: win32.sys PAGE_FAULT_IN_Non_PAGED AREA, with the advice to remove any recent driver upgrades. I tried to use System Restore to go back to April 23 when I installed an 'unsigned' driver but Sys Restore function doesn't seem to work. Therefore using DriverMax itself I'm starting to roll-back any recent driver updates, beginning with the Intel Q35 one.
Any further suggestions much appreciated please.