If you change-over your M/b and CPU, you will then need to
Repair Windows
Your new Motherboard, must be compatible with your OS (not all drivers are available for all versions of Windows, for all Motherboards)
Many Harddrives now come with the OS on a recovery partition
But this Recovery image was originally intended for the original hardware.
Therefore if the Motherboard and CPU are different to the originally installed hardware, it may not work (actually highly likely it won't)
If you are deciding on changing the Motherboard and CPU, you are best to have an original Windows setup disc (That is compatible with your hardware)
Your COA Windows key, may be OEM. Which means that it will not legally be allowed to be placed on different hardware.
There are many issues here, I'm not sure I can discuss all of them in detail