I have added a HD from my previous computer. It has Win XP Pro on it and I tried to use BCD to be able to access it. The boot loader shows both; however, when I select XP it just starts all over again. I can go into Win 7 fine.
You can't just use an OS from one PC on another, unless the hardware is practically identical.
All of the drivers and the configuration map in the registry of the old system will fail to correspond with the environment it now finds itself in.
At the very least you must "repair install" XP so that it can synchronize itself with its new environment. If your XP CD is pre SP2, this will also probably fail for lack of SATA support.
If the repair is successful, the new installation will need activation.
If the donor PC came with XP pre-installed, it will have an OEM serial number. The EULA for OEM copies of Windows prohibits use of the system on anything but the hardware it came with, so MS will decline to validate any attempt at activation on new hardware.