Ubuntu 12.04 boot disk not being read

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http://askubuntu.com – I have a copy of Ubuntu (11.1 and 12.04), both on CD and on USB key, but I am unable to boot from any of them. The CD drive in the machine is fine and will happily read Windows and Acronis boot disks, but tells me 'DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER'. It's a fairly old machine (Celeron 2.0GHz with 512MB RAM), but I figure that even if Ubuntu doesn't run on those sepc's, the boot disk should at least work?! Can anyone give me any suggestions as to why this would not work? Thanks. (HowTos)