Hello Chunkles, welcome to Seven Forums!
You will need to borrow or download installation media of the exact same Windows 7 version as the activation key you have so you can do a clean install
using that Windows activation key; it is against forum policy to post download links to MS software.
There's a lot of useful information in this tutorial linked below and be sure to post back with any further questions you may have and to keep us informed.
Clean Install : Factory COA Activation Key