Building a new gaming PC and I don't have a Windows licence. Where is the best, cheapest place to purchase a Windows 8.1 key? I've spotted a few on Amazon in the region of £40 which is managable, but they don't come with a disk and I'm not sure how to go about getting the actual software ready to install on a brand-new build (i.e. not updating from an existing pc).
The only other computers I have available are my Mac laptop and my girlfriend's Windows 8 pc (but I don't want to use her licence or anything).