Computer will not boot after i change hard drive.

I have a computer that was running windows vista. I replaced the hard drive with one that was in another computer that the power supply failed on. This other one has windows 7 operating system installed but it wouldn’t boot. The hard drive was fine before the power supply failed. It begins to boot then ask if i want to run windows repair. I select the repair option but it still won’t boot. What could the problem be?