You cannot boot the old drive (with XP installed) on the new computer. The drivers needed for the new hardware (chipset, processor, devices, etc) are not there (as part of the old XP configuration). Attempting to boot the old XP drive will fail with a blue screen.
If you want to run XP (which you should not be doing), you will need to do a fresh install of XP, and that is only possible if you can get drivers for XP for your new motherboard and all related devices.
Your best approach would be to do a fresh install of Windows 7.
Also, pay attention to the IDE/AHCI setting on the new motherboard's BIOS. If the old XP system was built with IDE, and you are attempting to boot on the new motherboard with the BIOS set to AHCI, you will also fail.
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