PowerBook G4 will only boot with COMMAND key pressed

My PowerBook G4 starts only when I hold down the COMMAND key at startup. This alone would not be such a problem, and it also was not for some time but now I'm running into the 'famous' setup assistant loop of Leopard and this is where the problem begins:

Holding down the COMMAND key at startup prevents me to hold down some other key, i.e. I cannot hold down the SHIFT key for booting in save mode, the 'C' key for starting from the installation CD or the OPTION key for selecting a start volume. I cannot even run the apple hardware test because of that.

In details what happens when NOT holding down the command key:
startup chime
screen remains black
after some seconds the HD seems to spin down and clicks
powerbook seems to be shut down again since I can repeat this

When holding down COMMAND key:
Everything seems to be normal (except for my bad system but this is another problem)

When holding down COMMAND + some other key(s):
startup chime
screen remains black
nothing happens, but the powerbook is still running
on releasing only the other keys except COMMAND, it continous starting up just like I never had holded down the other keys
on releasing all keys, HD seems to spin down and click, and pb seems to be shut down.

I hope someone has a solution to this, I cannot escape from the setup assistant loop and the laptop is not usable like that...

Message was edited by: troplin