"Recycling" parts speaks of reusing rather than replacing them. So I gather the mainboard will be the same one you use now, which has an activated installation, and just the windows installation itself will be new ? Under this scenario, the new installation will be activated .If your current build of windows is activated then a clean install will activate automatically. You don't need a licence key for the install process. Windows activation servers track the mainboard primarily . So even if the original installation was an oem, the board should remain activated after a new install so long as you have installed the same version of windows.