Can't use OSX install cd's

[schnatstik]

I've had this laptop for about four months now, my brother bought it used from someone and all it had was an unknown password and the powercord- no cds where provided at all. I was able to remove the password and use it quite well without any problems. Then the system got corrupt and I ended up buying Ibook G4 mac os x install discs, restore discs and the hardware test disc, however the only thing that worked were the hardware test. When I use the install discs, I turn on the laptop, insert the disc, it hums for a bit then after a few seconds it spits it back out. When I use the restore discs, it keeps them in, but it won't read them. And the hardware disc works perfectly. From what I can tell, these discs are cds since they don't mention anything about being dvds on their labels and I've been able to use the ubuntu installation disc to install linux. I really don't know what to do, can anyone help me?