Thanks lanxxx. I did what you said and now I managed to do the "cd /usr/standalone..." routine. The problem is that I still can't boot. If I boot without the DVD notinh happens. If I boot with the DVD in the drive but don't press f8 it will start booting from the HDD. First I get the white Apple screen, then there is a quick flash of the "multicolored ball" and then a gray screen then black then gray then black and so on, i.e. nothing else happens.
Any suggestions how to continue?