Even if you do get the drive to be recognized correctly, which I am going to say has something to do with the way you are hooking them up, are these IDE drives?
If so, do you have the jumpers set correctly on them? Do you have them plugged into the correct places on the cable? Are they SATA? Do you have the boot order set correctly for the way you are installing the new drives into the HP? A little more info would be helpful...but anyway.......
getting back to where I was going, you cannot simply move a Windows installion from PC to PC. It just won't work, even if you do get the PC to find the bootable drive with Windows installed on it. All the drivers, motherboard resources, everything about the Windows install is configured for your old PC, and nothing about the install is correct for the PC you are trying to put the drive into.
Not saying that it cannot be done, but it involves doing a repair install first on the new system, because it likely will never boot and simply blue screen, and then you can probably get it to boot, but the install will be flakey, and simply won't ever work 100% like it should.