The "OPERATING SYSTEM NOT FOUND ERROR" is easily explainable, but unfortunately for you not pleasantly:
Older IDE hard disks had a problem in which the platters of the disk would expand and contract as dictated by the laws of science when it pertains to metal substances. The problem arise that over time, the disk's platters expanding and contracting again and again would jam up the stylus, or the little needle that actualy reads the data from the disk itself. A common "quick fix" for this would be to bang/shake the disk, thereby freeing up the stylus. You would then be able to see the data on the drive (boot). This was only a temporary fix, and eventually the stylus mechanism would break, or the drive would fail completely, requiring a replacement.
Lucky for you, hard disks are cheap. Good luck finding one that works with your computer, however, as I suspect it is hindered by the accursed 2.0 GB LBA barrier.