Saerch the Apple tech info library.
Believe it or not they have the answer to this problem.
You must have certian extensions enabled, some of which are not labeled as such.
Here are a few that I can remember:
ISO9660Access (I think)
Some are too weird to remember but the TIL should list them all.
I had this problem on my dad's powerbook & the TIL had us up & running in 5 minutes or so.