As long as your XP Pro license is a RETAIL or UPGRADE licence, youi just need to start the install and call Microsoft for an activation code. With a RETAIL or UPGRADE license just tell the rep you are installing on a new machine and have removed the software from the old machine and they should give you an activation code with no difficulty.

If you have an OEM license that is tied to that motherboard, you can call and likely will get an activation; the odds of the SPA stopping by to check that out is close to zero. So the direct financial impact of reusing an OEM license is probably zero, however such a use is clearly NOT within the terms of the license. Up to the OP to decide if he/she is willing to violate that agreement however one-sided it may be.