Hello heier,
OEM software is only licened to run on the first computer it is installed on. Since a computer is just a bunch of interchangable parts, one of those parts had to be designated as the part that defines that unique computer.
The motherboard is the designated part that defines a unique computer. When a motherboard is replaced in a computer, a new Computer is created.
Your OEM copy of XP Pro is not licensed to run (and will not activate) on the new computer you created when you replaced the Motherboard.
Thank you,
Darin MS