PowerBook G4 doesnt boot but in safe mode

My old PowerBook was working fine - but a bit slow, of course… until yesterday. Since, I have terrible problems which make it unusable:

- a normal boot starts with the folder icon, with question mark then finder icon inside. Afterwards, the Apple logo loads, then a blue screen, then it freezes.

- I've tried to reset PRAM, PMU, verbose mode but it doesn't change anything.

- I've managed to boot successfully in Safe mode. Although very slow and with its inherent limitations, the computer works. But the disk utility cannot do anything (repair disk or disk permissions), both operations freeze the application.

- More strangely, I cannot boot the computer from anything else: USB key, external hard drive or DVD with each one Leopard Installer do not work. The installer won't load after the startup, even when I choose manually the drive.

So I'm really worried about this mess, and I don't see how I could get back the computer to life. Which I really need.