Thanks for this very helpful response.
I did try a BIOS update, but as the laptop is so old, it does not have the boot from USB facility, only from floppy disk and I'm not sure how many you need to install Windows XP...
I think I will avoid fiddling with the internal circuitry and (reluctantly) see if I can find a replacement DVD-ROM.
BTW - the laptop is a Toshiba Portege 4000.