a little elaboration on OEM. There are differing kinds of OEM. If, for instance, your PC is HP or Dell etc, then the OEM windows is tied to the motherboard as stated above. In this case you'd need a new copy and a key.
If you had your PC built by a custom builder such as iBuypower, it will come with an OEM copy of windows pre-installed and you own the CD. This type of OEM is a full retail copy unlike the Dell/HP proprietary copy,and as such is tied directly to the key. In this case, you'd just have to read the key from the case or booklet when installing windows, and you'll be all good