I have a 2005 iMac G5 17" ALS (Ambient Light Sensor) that will not boot. When I attempt to boot from the hard drive, the gray screen with the Apple logo and spinning icon appears, followed by the blue screen, and then it freezes with a spinning beach ball on the blue screen. I've tried to boot from a CD/DVD, using both the original Tiger OS disk that came with the machine and the Leopard install disk that was used to upgrade it. When I try holding the Command key while booting, the startup manager opens, displaying the HD as an option. But when I insert the disk, it spins around for a while (with the cursor a timer icon), and eventually just ejects the disk without ever showing it as a boot option. Clicking on the refresh button does not help. When I try holding the C key while booting (with the disk already in the drive), it spins around while displaying the gray screen (without the Apple logo), and then ejects the disk. After ejecting, it attempts to boot from the HD (gray Apple logo screen, followed by freezing on the blue screen). Other details: I've reset the SMU (by removing all cables, then reconnecting the power while holding down the power button). I've reset the PRAM and NVRAM (using Command+Option+P+R). I've switched the RAM modules. I've tried unplugging all peripherals. Internal diagnostic LEDs 1, 2, and 3 all indicate normal function. What does it mean when it just ejects the CDs/DVDs like this? Does that indicate a logic board problem? What else can I try?