1. These computers have the OS installed to those hard drives and a new drive would do nothing. Your post does not reveal how you prepared the HDD for booting.
2. The CAR analogy. Some folk think you can change a HDD like you do a car tire. Sorry no. It's much more complicated.
I just removed a very full internal hard drive (Seagate IDE) and replaced it with a new 160gb (Western Digital) hard drive. I thought it would be a simple swap, just needing to change jumpers as I was slaving the old drive. However, none of the drives are booting up I am getting a clicking sound from both drives. What went wrong we simply unplugged and followed the directions that came with the new drive? Windows XP O/S.