Why would a PC not boot from CD?

DVD actually. I've taken the DVD player out of one machine and put it into
one I want to install Small Business Server on, and although I can tell the
BIOS to boot first from the CD/DVD, it fails to find the boot record,
although I get a message to say it's looking for it on the CD. I've also
tried it with an XP Pro CD, and the same happens. On all other machines,
the XP disc is bootable. What am I missing? I'm no great shakes at the
BIOS level - it's all half (quarter?) understood.

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## PH, London
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