iMac not starting up?

I have checked several forums on this but none of them has helped me properly...

My old iMac (leopard OSX 10.5) suddenly crashed. I turned it off via the power button and then restarted it. There's a strange file with a question mark as it tries to start up.

1. I've already tried pressing the options button - no disk turns up, its all blank but I can see my cursor.

2. I've tried putting in the Install Disk but nothing has worked

3. Already tried pressing 'c', 'shift' and 'option-command-p-r' but none of it has worked.

Thanks

So, pressing c and holding it until you can tell the computer is using the Install DVD hasn't worked?
Except for this, it sounds like just a dead hard drive. If it won't start with the correct Install DVD, then some other part of the computer is also dead (probably cdrom drive, but who knows?).

Sounds like what mine did 2 days ago. Took it in to the apple store today and was told that its not getting enough power to start up. Which is a sign of a dead board.

They gave me some places that could fix it for about $700, but I'll probably just get a new computer and try to sell this one for parts.