I would try these in order, until you can get it fixed (warning - I am a PC hobbyist, not a computer professional)

1. I assume you booted Knoppix from CD? If so, couldn't have done anything permanent to the system from what you describe. I can only assume your problem comes from somewhere else. If you installed knoppix, sounds like you might have hosed your MBR. There's a sticky at the top of one of these forums telling you how to "ungoof your MBR" - you might look for it and see if it helps. Again, don't worry about this if you booted from CD.

2. Couldn't tell from your description - Knoppix on your PC, or the other?

3. (Windows) Can you get into scandisk or safe mode? If not, look at BIOS settings. If scandisk doesn't fix, try booting into safe mode

4. If you can boot into safe mode, see if you can do a system restore to your last restore point

5. If you can get into safe mode, then run msconfig from a run prompt. Temporarily shut down all non-essential services, and do a selective reboot. If it boots, look at the services you shut down, and re-enable one at a time until you find the offending service.

jd