There are two common problems that might explain your problem.

The most common is a bad burn as a result of burning at high speed. If you didn't burn the disc at a slow speed, go back and burn another one slowly (I suggest 4x for CD's, or as slow as your drive and software will let you go if you can't do 4x). This really makes a difference and we have documented many cases of it. Burning the disc on a RW media that has to be burnt slowly also usually resolves this problem.

Another potential problem is the dma/nodma issue. 4.0.2 boot with dma off by default, but turning it on with the cheat code knoppix dma can make a lot of difference on some systems. Some previous versions did default to dma on and on those if problems were see you needed to use the knoppix nodma cheat code.