After almost three weeks of trying various tools, etc., I've finally managed to repair the damaged boot sector. Thank you very much to to Klaus and Costakis for trying to help. None of the linux tools I tried worked. Manually trying to fix the partition table didn't work either. Eventually, I found a Windows tool that fixed it in place. It appears that I have everthing back to normal with all my files and data intact.
As far as I can tell, whatever was written over my boot sector wasn't the knoppix USB stick stuff. I could be wrong about that, but to my insufficiently knowledgeable eye, it didn't seem to be that. I still don't know what did it. The drive seems to be fine.
I apologize for my slow responses on this matter. Of course, the dead drive had my email on it. Emailing by phone is a pain. 8(
Needless to say, I have backed up my drive again (raw and data.) I had backups of a lot of it, but it would have been a big pain to restore and I would have lost a lot of recent stuff. We all know to backup regularly, but I let it go too long...
Thanks again to all who tried to help.
P.S. Even though the linux tools didn't fix the problem, Knoppix was of great help in backing everything up and trying to solve this problem.