|The "big grey ones" means the drives are IDE/PATA rather than SATA. Therefore there is some setup involved. Hopefully you know that there are generally two IDE channels - primary & 2ndary. Each channel can accept 2 drives - master & slave. You can NOT have 2 masters or 2 slaves on the same cable/channel.|
On the end of your IDE hard drives (& CD/DVD drives) there are small jumper caps that have to be moved to designate the drive as master or slave. There's usually a diagram on the label or there may be marking near the pins (MA, SL, CS). Configure the drive you want to boot of off as primary master.
But be aware that XP doesn't like being moved from one system to another. You *may* get lucky & be able to boot into Windows, but more than likely you will just get a blue screen.