I fixed one computer by running the instructions that Dwight listed to get into the Recovery Console, with one change. Once I had done the 5th command in the list, I typed fixmbr instead of the attrib command listed. That was the first time I had ever gone into the Recovery Console. I didn't have any resource materials with me. I was expecting a menu based console and didn't know what to do. Finally, out of desperation, I typed "help". That got me the list of possible programs to run. I chose the one best matching the symptoms, based on previous experience. I was lucky.

If I recall correctly, the symptoms were identical to those in the original post. It seems to me that an ntldr error is a catch-all error message similar to the old message I had grown to "love" saying "non-system disk".

While a failing hard drive can cause this type of problem, I also believe there are other causes. A system crash, or a power blip at exactly the wrong time could do it as well.