I used a retail OS X 10.5 DVD to "erase and install" OS X on my PPC G4. It had been running OS X 10.5 without problems before but I wanted to erase everything on it. (In fact, I believe I upgraded it 10.5 with the same DVD). However, the mac crashed about 80% of the way through the process. It displays an alternating sad mac and question mark when I boot it up. I am occasionally able to boot it up from the CD but there is always a kernel panic before I can click the install button. What should I do? Thanks.