Hey Butters, you do know that you have just violated the EULA several times and you really have no right of using Windows anymore, unless you uninstall Windows of every single machine apart from the one that you installed Windows on originally. It goes back to the concept of, just because you can physically do something, does not make it right (Legal).
When you bought your copy of Windows (OEM), you should have really been completely aware of the licensing terms of an OEM license, which is; if you have the OEM version of Windows, you can change any system component in your machine apart from the motherboard and you will still be licensed. You may have to re-activate Windows after a hardware change but you will still be allowed to activate and will not be breaking the EULA. Now depending how significant the hardware change is, for example updating the BIOS of your motherboard, you may have to ring Microsoft up yourself because online activation may fail. If you do have to ring them up, just tell them what has happened and they will happily activate your copy of Windows Vista for you. If you change your motherboard for a performance upgrade, then you will have to go out and purchase a new copy of Windows (New License).
There is another, often misconception regarding being activated and being licensed. There is a slight but a very important difference between being activated and being licensed. If you have a copy of Windows OEM and change your motherboard due to a performance upgrade, as said above, you will have to go out and purchase a new copy of Windows. If you do not wish to do this and use your existing copy of Windows with your new motherboard, the only way you will be able activate Windows you already have, is to ring Microsoft up and lie to them to get them to reactivate Windows for you, if they give you a new code, you may now be activated but you are certainly no longer licensed and would have just broken the EULA. This is because, Microsoft gave you a new code because you had lied to them, if you said the truth and told them you changed your motherboard for a performance upgrade, they will tell you to go out and purchase a new copy of Windows. Microsoft give's it's customers a certain amount of trust concerning the licensing terms and if you wish to abuse this, that is up to you.
Now I am not sure how you have been able to reactivate Windows over the internet so many times that you have done but effectively, it's the same as ringing Microsoft up and lying to them. The point really being is, you shouldn't even be trying this in the first place.
but I have used the same OEM version of XP Pro in around 8 different builds.
In my opinion, that really is taking the mick and I can't actually believe you have used the same license on so many machines.
If you wish to dispute anything I have said Butters or anyone else for that matter, that's fair enough but please don't turn this into a personal insults thread. I am quite a morally person myself and I just cant stand or as a matter of fact sit

here and say nothing.
