Is the PC in question a branded PC like Dell, HP, Gateway? If so it's an OEM licences, which means that copy of window is tied directly to that mobo. You would need a new copy of windows...OEM or retail. I recommend getting the retail version and the Pro version. It's a bit more but the best thing about it is, it's transferable, meaning you can move it from PC to PC or upgrade mobo to upgrade mobo. It can only be installed on one system at a time.
http://www.newegg.com/Operating-Systems/SubCategory/ID-368
Here you can see the price difference between the two version on windows 8.1 pro oem vs retail on the right side of the page.