Okay, the copy of Windows that came with the computer is called an OEM copy and is tied to the machine it came from. It cannot be used on any other legally.
You can buy a retail copy (called retail or FRR) and it can be used on the mac. Some places also sell OEM copies that are meant to be purchased and installed on machines people build. Technically you can use that if you buy some sort of hardware with it (flash drive...) and use that for your mac.