iMac wont startup and won't boot from CD My 24" 3.06Ghz iMac will no longer fully boot. It constantly gives me the grey "Your machine needs to be restarted" half way through booting. At first I was able to boot into SuperUser mode and run an fsck which passed normally, but eventually after the constant rebooting fsk now reports:

Invalid node structure
The volume Macintosh HD could not be verified completely


I have also tried booting to my Snow Leopard install DVD but that just brings up the grey "Your machine needs to be restarted" screen. 

I assume now that my HD is toast (When using another Mac, I am unable to mount it in Target disk mode), but the inability to boot to the CD tells me there is something deeper at fault here. Is there any way that I can further diagnose the problem?
From what you describe, your internal hard drive failed, which will require a replacement;  since you have an iMac, I suggest that unless you’re very comfortable using a variety of tools and have time, let Apple do it for you. In any case, what you can try to do is, connect an external USB/FW drive and try to install into that drive. 

The fact that your drive doesn’t want to boot from the DVD is intriguing if anything, but the DVD may be crashing when reading the “broken” drive. It’s rare, but it could happen. Do you have any other Mac to “clone” into an external drive and try to boot from there? 

Since it’s an iMac, your best bet is to remove the internal drive and proceed from there. The problem is that removing the drive is a painful operation. 

Best idea: go to an Apple store or certified shop. They have the tools and the spare parts to better diagnose the problem.