Any Windows XP that comes bundled with OEM computer such as Gateway, Dell, HP, etc is most likely a OEM Windows. In most cases you cannot install Windows XP on a system that does not have OEM identification in its BIOS, if you move hard drive to another system, you cannot activate. This means that you must order your new motherboard through your OEM manufacturer if you want to get a new motherboard, they rarely give you such possibility though and instead want you to buy a new computer from them.
In other words, as long as you use the original motherboard that came with the OEM computer you bought, you will be fine. But if you buy a new motherboard, you have to buy a new Windows as well since OEM identification is in motherboard BIOS.