stubbornness :
i want to change mobo+cpu but i need to re install windows, will doing that work, or will i need to boot from usb because my pc didnt come with a disk, i have the product key, i just need to know how to reinstall windows on a store bought pc.
You can only use the same motherboard in the system, two things will go wrong if you try.
Windows will need to be purchased again since it's locked to the system it was installed on. It would be an OEM vesion supported by the PC vendor it came with.
The factory restore will likely fail if it detects a non-standard motherboard or it will not boot becuase of the driver differences if it does install all the way.
You would need to buy a new copy of Windows 7, or contact the PC vendor and see what can be done, although if you are swapping motherboards yourself they likely won't help you much.