It sounded like you were trying to install Knoppix to the hard disk in a dual boot configuration, that's why people usually ask about replacing the NT loader. If you are just trying to get Windows working again, I would suggest salvaging any needed data with Knoppix and then reinstalling Windows from the Microsoft disks provided. Knoppx (and Linux) does not include the NT loader, that Microsoft code. And as far as I'm concerned, Knoppix can safely read a NTFS partition but should not write to it.

As to the NT loader, that's Microsoft code that starts Windows. A very small part of it is in sector zero, also called the Master Boot Record or just the MBR. Because of space limitations, this is just enough code to call in the rest of the loader from the Windows partition and load windows. In the unlikely event that only the MBR is damaged, you might be able to install one of the Linux boot loaders such as grub or lilo. They know how to call the rest of the boot code in Windows also. But I expect that the actual NT loader on the windows partition is what is missing, and that being Microsoft code it's not part of Linux and Linux can't replace it.

So your best bet as I see it, is data recovery with Knoppix and then one of the all too common reinstalls of Windows (unless your friend wants to give up on Windows and install Linux).