My iMac will not complete a boot, what are my options?

Recently, I had been using a complete shut down due to the machine going completely asleep. This seemed to give me usage as desired. Several days ago while booting from shut down, the boot would not complete. The spinning pie would appear in the top left corner. Wallpaper is accurate, dock is showing. Tool bar is NOT showing.

I have tried to boot in safe mode (shift during start-up) and the machine shuts down. I have (per online suggestion) inserted the OS X install disc (OS X 10.4) with no results other than sounds to indicate activity in the hard drive / disc reader.

This machine is approximately 10 years old (or older) and has been a strong performer until now. What can I do?

BTW, I am NOT a technical genius. I request any assistance to be in the simplest terms...

Thanks