Ironically enough you would still need a running computer to download it to. The download is in the form of an iso disc image which you can either burn directly to DVD or extract to a usb drive. If you're using the usb drive there are a couple more steps but google's a god for that.
You are buying the key, it's up to you what you choose to do with the disc or digital download that comes with it. However, Microsoft have got to assume you don't already have installation media.