Hello FrankJH, as stoofa has already said, what you can physically do and what you can do legally are two separate things. As well as, you may be activated but that doesn't necessary mean that you are licensed.
If you go out and purchase the OEM license of Windows, you should know and fully understand the licensing terms of that particular license being; You can change any system component in your system as many times as you wish apart from the motherboard and you will still be licensed. Now you may have to re-activate Windows after a hardware and/or configuration change either by the online process or if it fails for various reasons, you will have to ring them up. If you do have to ring them up and speak with a Microsoft Agent, just tell them the truth, what exactly has happened and they will happily re-activate your copy of Windows for you. You will not be violating the EULA and you are still within your rights to use Windows.
Now if you change your motherboard for a performance upgrade (We are not talking about replacing your motherboard under warranty), then I am afraid you will have to purchase a new copy of Windows (License). If you wish to use the same copy of Windows with your brand new motherboard, the only way you are going to get Windows re-activated is if you ring up Microsoft and lie to them. You now may be activated but you are certainly no longer licensed and would have just broken the EULA.
You now may say that if Microsoft themselves activated Windows Vista for me, how can I off just violated the terms of the EULA? They gave you a new code because you had lied to them, if you had said the truth that you changed your motherboard for a performance upgrade, they will then say to you, you will have to go out and purchase a new copy of Windows. Their is a certain amount of trust Microsoft give its customers concerning this and if you wish to abuse this then that is up to you.
