I assume you are trying to push the button on the front of the machine right after power up. (Keyboard eject does not work on the AIO IIRC). If you wait until it thinks it is booting from the CD then there is no way to stop it short of restarting.

Since you have tried two CDs I'll throw out the off-chance that they both are bad or both have CDs stuck in them.

Check that the hd is mounted underneath the MB carriage and and attached to the ATA cable there. The CD is connected to the other ata buss with a single device cable. If a two device cable it should be connected to the master side to boot. (assuming your CD is not SCSI). CD eject should have nothing to do with the HD, but these things will help you load/boot the os

But it may be something even more odd. These machines have a two-part button that sticks outside of the case and is supposed to contact the eject button on the cd. If these are not lined up, or the inner part has fallen off, this contact will not be made and the CD will not get the message. I have, in the past, just removed this button all together and used a screw driver to reach in the hole and press the button on the CD.

Good luck. The AIO is a often miss-understood but lovely machine.

JHV

p.s. if you have not done so, read up on the "8 gig limit" issue.