Conclusion: I couldn't get Knoppix to work either. This is an old machine which never worked right -- very poorly made. Today I bought a Gigabit Ethernet and everything works fine. It was the NIC card, as you all suspected.
Yeah if it didn't work with Linux then I would have agreed about the hardware. I'm glad that you got it straightened out. Linux checks ethernet cards during boot up which might be why Ubuntu crashed.