Sorry guys, my mistake. I had inserted the cd from a Mac Mini. Here at my workplace we have all the cds for 18 Macs (among which are Powerbooks, Mac Minis, Power PCs, iMacs.....) in 1 closet

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So I rechecked the cd and it was not for the Powerbook, I had to go in another closet to search for the Powerbook box where I found the right installation discs.
So, problem solved. Too bad I wasn't able to figure later that the cd was not the Powerbook's, the computer was just stoping at startup during boot.
Thank you for your help anyway.