Can't boot from a CD?

Short version: Make sure the IDE controller that the CD drive is attached to is actually enabled.

The BIOS on my ASUS motherboard allowed me to choose 'ATAPI CD Drive' as a boot device, and on boot, no matter what CD (or CD drive) I tried, the drive would make a few noises, but the system would not boot from it. It turns out that the ATAPI device I had selected was actually some sort of dummy device; the system wasn't even trying to boot from the CD I had put in, because the Marvell IDE/PATA controller the CD drive was connected through was not actually enabled. With the controller enabled (somewhere in BIOS), there was a new, 'real' CD drive to select as a boot device, which of course worked straight away.

Asus EZ Flash "ROM ID in the file is incompatible with with existing BIOS"

Short version: Burn the new rom to CD, or try a different USB key.

I found a lot of threads about this, but none had my eventual solution. I tried to update my BIOS with every version they had on the ASUS website using the EZ Flash utility in the BIOS tools menu, having transferred them all to a USB mass storage device. Every one of them gave a "ROM ID in the file is incompatible with existing BIOS" error. After probably two hours of dicking around trying to get different DOS boot CDs to work, in a vain effort to try to use the AFUDOS to perform the update, I eventually discovered that those same images, when burnt to a CD, could be used by EZ Flash without issue.