My father in law bought himself a new imac G5 with Tiger . It came with the install /restore discs and he wanted me to install it on his older G3 400 imac but it would not install the OS on the old system - can I install it is there something that I am missing or I simply can't do it from these discs ?
Yes, correct. The software included with new computers is system specific and therefore cannot be installed on other computers. Even if it wasn't system specific, it would still be illegal