I am looking to build a new system using a lot of the parts I have in my current PC but if I change the CPU will that make the computer think it's new or will I have to change the motherboard as well? It needs to at least think it's new because I have had several problems with this computer and I don't wanna risk them coming back when I reinstall my OS (it's retail). I am planning on getting a new CPU, GPU, PSU and case but everything else I plan on re-using.
Windows operating systems are hardware specific, so when changing the CPU it will make you reactivate the OS or in worse case not even boot into windows. However changing the motherboard might not affect it. I recently(this weekend) replaced a motherboard from HP factory motherboard to a ASRock N68-C, and changed the CPU(same CPU higher frequency) and i didn't have to reinstall. But Changing say from I3 to I5 or I7(or Phantom to FX) will most likely not boot into windows and make you have to reinstall windows and reactivate it. Since it's retail you should be able to use the same key no problem.