XP will most likely fail to boot in a new system since the drivers are for your other system. A clean install is recommended when switching out the motherboard/system.
Also, to dual boot effectively, XP would need installed before win 7. There are ways around this. You can google for them.
If you have the option to boot off selected devices during your bios boot, like "F-12: Boot" or however you are presented with this option, you can try putting the XP drive in the new system, disconnect your current drive(s) and try to repair install XP. Once this is done, reattach all drives, then just use F12 at startup to choose the boot disk.