If a Knoppix CD booted, then the MemTest CD should also. You can create a MemTest Only CD instead of a UBCD that you may be able to get to boot. If it won't, then you have hardware issues. Try disconnecting the hard drive and use only the CD drive to boot and run MemTest. If that works OK, then it is a drive problem. If it doesn't, then you may have a CD drive, IDE Controller, memory, or motherboard problem.