well if ur going to do any or all of those, you should backup wateva u want to keep.
1) backup all the stuff that you want on your pc.
2) install your new motherboard.
3) go into your bios and select the first boot device to be your optical drive (eg. cd-rom) put your windows cd in your computer and reboot. boot to the windows cd.
4) while setting up windows to install, its going to ask you which partition you want to install windows on. delete your current partition, recreate a new partition using maximum space, and format using ntfs full.
5) continue with installing windows.