If it is an OEM Windows installation you will not be able to activate it with a new motherboard, that is outside the license.
If you are using the same motherboard and it is Windows 7 there should be a sticker on your machine with an activation code -- write it down. Then download an image of the same Windows 7 version from
HERE and burn it to a DVD. You can use it to reinstall your OS and then re-activate with your key.
If it is XP, Vista, or Windows 8 your options are much more limited. Some OEM builds come with a recovery partition and you can make recovery disks from them.