I too have exactly the same problem.

Have not used Knoppix for a very long time or any linux for that mater. But decided to come back and have a look. Unfortunatly I get the same thing, just freezes with a black screen when it gets to loading the X system stuff. I have tried the CD and the DVD version and both with the same result. Tis a shame because I tried it at work and it all goes fine and was thinking of setting up my older computer with knoppix. Will try what you posted to the other user.

BTW my graphics card is also a radeon, bit its a 9600 pro, and it is picked up as that during boot.

Anyone that can shed some ligth on it, I would be gratefull.