This happened to me, and yes, it concerns that hard drive. I've seen it on my XP Pro installations, but it was only when I added a slave, or misplaced a jumper on the master. Namely, if this happened without anyone's intervention, I don't really see how it happened.
If you make it a slave drive on a different computer you will have access to all of the drive... as long as the other computer is running NTFS (Win NT, 2000, XP, 2003)
Your hard drive may be failing. I emphasize may. At any rate the ntldr.dll file is critical to the system starting up as it is part of the operating system. Once you get your files that you need off of the drive I would suggest re-formatting the drive while it is still on the other machine and then re-installing the operating system after putting the drive back into the original computer. If the problem was created by a bad sector on the drive... reformatting will fix this.