I have successfully installed a new motherboard and processor on numerous occasions by starting the OS into safe mode on first boot (F8 during post to make the boot menu appear). Whilst in safe mode remove all drivers related to the motherboard, processor, chipset and graphics (and any other item that relies on motherboard resources) and then restart normally. Windows will then require you to install all the new drivers. I would stress, however, that I usually use this as a stopgap measure after a board failure to get data from applications backed up before doing a clean install of the OS. That said, I have had several machines that have run quite happily without a re-install for many months afterwards until I could get around to doing a fresh install.
Sometimes windows has wanted to re-activate and sometimes not. I think it depends how drastic a change has occurred, e.g.generational change in processor or chipset would re-activate but a simple board replacement doesn't.
Good Luck
Q