If you've just taken an old XP system out of a previous PC, plugged the HDD into new hardware, and tried to dual-boot it, the OS is never going to run without a repair install.
If the old PC HDD is IDE and the new PC has SATA, then you'll need an SP2 version of XP for the install, or else get SATA drivers from the mobo site and F6 them into the install from a floppy before setup runs.
If the old PC was an OEM (not self-build) which came with XP pre-installed, then the serial number will prevent it ever being used on another PC, because the OEM EULA ties it permanently to the original hardware it was supplied with. Even if you repair-install it, MS will decline to validate your attempt at activation.