Installing Knoppix in any virtual machine is just as bad of an idea as installing it in a real machine, if not worse. If vitrualbox supports mounting ISO files as if they were optical drives, like VMWare does, then I suggest that you use that approach rather than "installing" Knoppix and the problems it presents. If you don't want to run from a read-only ISO image then I suggest installing Debian Etch or some other Linux distro suitable for hard disk install into your virtual machine, not Knoppix.