If you replace your board with the exact same model configured with the same BIOS settings then you don't need to re-install Windows. Something like that should be almost completely transparent to the operating system. Your data (files) won't be impacted.
If you replace the board with the same model but configure the BIOS differently or if you replace it with a similar model (but same chipset), Windows may detect a hardware change and ask you to re-activate. You shouldn't need to re-install Windows. Your data shouldn't be impacted.
<edit> The only time this could pose an issue that I can think of is if you change your SATA ports from IDE/Legacy mode to AHCI or vice versa. Windows won't be able to boot if it was installed in one mode and you tried to boot in the other. If you screw up the repair process you CAN end up accidentally wiping your data. The best way to avoid this is to try the other mode and your PC should start up without any issues. </edit>
If you replace the board (and CPU) with one of a different microarchitecture (replacing (z68) Sandybridge with (Z97) Haswell) you will probably need to re-install Windows. Even if you don't need to, it's highly recommended that you do. Do make sure you back up your data before proceeding.
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