Sift through this first! http://www.tomshardware.co.uk/forum/51259-47-windows
If you change your motherboard you will almost certainly need to clean install Windows and re-install your Programs. As for re-activating, the above thread will tell you almost all you need to know about OEM...
Your copy of Windows is locked to the PC on which you purchased it. You cannot transfer that license to another PC.
You can upgrade any components or peripherals on your PC and keep your license intact. You can replace the motherboard with an identical model or an equivalent model from the OEM if it fails. However, if you personally replace or upgrade the motherboard, your OEM Windows license is null and void.
Two more questions . If I buy the retail version of windows, can I change the motherboard without having to reinstall? Also if Microsoft gives you a new activation code, will all the programs stay?
Clean install is recommended purely as it gives rise to less conflicts and hassles caused by old drivers. Once installed, Windows needs to be activated, hence the need for a new key. Activation does not affect installed programs. Activation has to be completed within 30 days of installation.