I have been a given an iMac 24" with a crashed Ubuntu installed on it. I have an install CD of Snow Leopard (not a machine dedicated one). I use a PC keyboard.
How can I install SL onto that machine to replace from scratch the installed OS ?
When I use the combination option-boot, or C-boot, or Option-R-boot, nothing happens… then Ubuntu launches and crashes. I never see the Apple logo.
When I insert the CD into the Mac at boot or before the boot, it keeps it a little time at boot, then ejects it.
EDIT 1
I have rebooted holding command-option-P-R. No display, the Mac makes THE sound, and auto-reboot.
Then I hold the option key: nothing happens, no image onscreen (power button of the screen is blinking), even if I wait 5 minutes. If I release the option key (alt on my PC keyboard, the Windows key should be the Command one), nothing more. If I then press the power button on the Mac, it shuts down instantly.
If I then reboot in a normal way, Ubuntu launches and crashes as usual.
TIP : I use a Dell 27" screen.
EDIT 2
I've burned a linux boot CD onto my W7 x64 PC (Ultimate Boot CD), inserted it into the booted Mac under crashed Ubuntu (no ejection of the CD, the drive tried to read things onto the CD), Hard reset, then kept the C key during boot. The CD drive did not tried to read the CD (no sound), and Ubuntu started/crashed as usual.
Best Answer
You may need to separate the boot process from the CD reading.
First, power off and reset the NVRAM by holding option + command + P + R
Then, I would boot with the option key held until you have a grey screen. Once the Mac is at the boot chooser (no apple logo yet), you can then insert the CD.
This is firmware running the Mac, so you don't even need an internal HD to mount the CD. If it ejects, you need a repair of the drive or a replacement CD.
If the Mac can't boot to the boot chooser, then you have a hardware issue on the Mac.
Looking over the further details, if your Mac can't boot to the grey screen then there is a hardware issue. It might be as simple as disconnecting a failing HDD or just reseating the HD cable. It also might just need you disconnecting every single peripheral and bringing a new keyboard to the hardware.
I'd seek assistance from AppleCare or a hardware tech if you can't get the boot picker to show up. You could also see if your Mac supports internet recovery, but some 24 inch iMacs were made before that was added to the firmware.