It's mixed results with XP, but Vista and Windows 7 are a lot better at accepting drastic hardware changes. As said if it boots you're laughing and you just need to activate again. If you have an OEM copy you'll need a new Windows license if you're upgrading the motherboard. If it blue screens at the Windows loading splash screen check the IDE/AHCI setting on your motherboard is correct.
Not sure what you mean about OSX and just reinstalling the system part, as you aren't going to be upgrading a Mac motherboard and just reinstalling the system files is called a repair install on Windows boxes.