I upgraded my Windows 7 machine a few months back to Windows 8 Pro when Microsoft had the deal for 30 euros. Now I've covered some problems and I need to reinstall. How do I do it when I only have Windows 7 disc?
The critical part is the product key that you already purchased. Now you need an install image or disc that will prompt you for that product key. If you don't have a friend that has an install disc you can borrow, then you can download an iso image from Microsoft as part of the upgrade assistant process. Here is a page that steps thru how to download and re-install windows 8. Be sure you pick the same bitness (32 or 64). Its a good idea to have a physical windows 8 install disc for any future repairs (like rebuilding the boot loader etc).