my pc keeps booting up then going into hibernation
after i plugged in my old hardrive from the knackered pc which was doing the same, so i could copy the files etc

If you're trying to boot from the old HDD then it's clearly something wrong with the OS/setup on that drive. Simply don't boot from it until you've formatted it/fixed it.

If that's not the case then just try removing the HDD again and see if the problem goes away. If it does, it's a bizarre hardware issue Ive never come across or heard of.

If it's still doing it after removing the old HDD then odds are you've transferred some virus/malware from one PC to another. Time to run a bunch of AV software.

Are you plugging the second drive into the right IDE slot on the motherboard? If you are running the second drive as a slave then don't forget that you need to change the jumper on the disc from 'master' to 'slave'.