MS need to stop the whole oem licence nonsense - mostly because they don't even adhere to the licence agreement themselves. They know that people use OEM windows for subsequent PC builds and they don't seem to care.
If MS really did enforce their OEM licence then they'd likely cause a massive drop in OEM sales.
I've used an OEM licence on a PC that changed a lot over a number of years and never experienced any problems. If MS allow you to activate, then they cannot say you are breaching the agreement - they have personally barred themselves from objecting.