Yesterday I once again encountered the XP glitch where suddenly your optical drive will disappear and not show in My Computer or even in the Device Manager. This has happened twice before and both times I was able to fix it by running a little program written by a guy named Doug Knox that deletes some registry entries relating to upper and lower filters for the CD drive. This time, that did not work. I even did a system restore back to before the drive disappeared and that didn't even fix it. here is what I figured out and did to get the drive back. It may prove useful to anyone else experiencing this problem.
I went to the device manager where no optical drive was showing. I then inspected the IDE controller and found that the Hard drive was on the primary channel but nothing was showing on the secondary one. The secondary channel was set to autodetect using DMA if possible. I changed this to use PIO mode only and then right clicked on the computer icon at the top of device manager and chose "scan for hardware changes". The screen flashed and my optical drive reappeared in device manager. I then changed the setting of the secondary IDE channel back to detect in DMA if possible and rebooted. Now the drive shows and is working and in Device manager it says it is using Multi-word DMA Mode 2. I guess when I had changed it to detect in PIO only it picked it up as strictly a ROM drive and was not initially able to detect it as a burner drive. It is an LG DVD burner that can use all formats including DVD-RAM and is the drive that came with the machine.
(There is no option anywhere in the BIOS of this laptop to set up detection of drives or even to show what drives are installed. The only options are to set the boot order and the options are just called Hard Disk, CD Rom, Floppy, etc. The BIOS has been greatly pared down by HP unfortunately. Probably to prevent people from messing things up.)