Okay - last one for tonight... have replaced motherboard/CPU and upgraded to XP. My DVD drive (a Toshiba SD-M1202) appears to work correctly - device manager reports no problems, I can play audio cds, view the contents of cd-roms/dvds and copy files from them.
BUT - if I try to INSTALL a program from a cd-rom or dvd in this drive, I get the message "the application failed to initialize properly (0xc0000005). Click on ok to terminate the application". I had to use my cd writer to install XP and all my software, and of course I can't install anything that's on a dvd. The drive worked fine with win98/old motherboard.