Do have to install.................

So you have to install a new os for a new cpu or mobo or bothDo have to install.................
I'm actually not sure since I've only built one computer, but if you use the same hard drive I think you just need to install new drivers.Do have to install.................
Usually you need to re-install the OS from scratch when you change the motherboard for a different make or chipset. Sometimes you can get a away with it and the machine boots up, this is rare in my experience. When the motherboard is a different make Windows doesn't boot, it will just keep rebooting. As long as the you haven't changed anything else in the system a CPU change doesn't require a reinstall of the OS.
Generally not for a new CPU, but I would recommend it for a new motherboard. The problem that comes about is that when you replace a motherboard, you a essentially changing the identity of the computer and the OS may or may not recognize the computer as being the same as it was before and will not work completely. Now in my experience, if you are using the same motherboard company (I used two different ASUS motherboards), the OS probably won't have any qualms about the new one...but just to make sure everything is compatible, I would reinstall the OS.Hope this helps.