Have you jumper set it to slave, or plug it in secondary IDE channel using seperate IDE cable? Please check hd configuration and verify that it is being properly detected in the BIOS...... 
If it can be detected in another computer, BACKUP your files as soon as possible, your hard drive IS probably failing...... 
Possible, but the information given just points to accidentally lost boot information.
The fact that the drive was detected as Primary Master in the original computer is not enough to boot up windows XP. the disk also has to have, say, windows XP on it (which it does) and the boot loader (which was missing).
The copies of ntdetect and ntldr need to be placed in C:\, as opposed to C:\windows. Not sure if this is where they ended up.
placing the drive into a different computer would require the movement of the jumper to indicate it is a slave, and, if the primary master present didn't previously have a slave drive, a possible jumper change in it as well (from Single Master to Master W/ slave present or something similiar).
did you try JJ 3000's suggestion?
