i have a harddrive that was in another computer i have, that will not boot that computer up when it is the only harddrive. but i can access the information on that harddrive, and store additional info, when it is set as a slave drive and put into another computer, that has another harddrive to boot from.
Do you have other devices connected to the same Parallel ATA cable? Do you use 80-ribbon cables instead of the older 40-ribbon cables?
Try running one device per cable, and use multiple PATA ports. Or buy something with SATA. Personally, i would give away old pcs and build a new basic pc from parts for less than 150 euro. Thats much more modern than the old stuff you have, and you can build pcs really cheap if you know the right parts.