Well, that's the hard part. You can either get a pirated copy and risk being identified for copyright violation, or buy a legit copy. It's interesting that Windows doesn't let you make a clean installation disk. You *must* copy the entire contents of your hard drive onto a backup installation image. I wonder why that is. After nuking the hard drive, you could install any OS you want. You could not install anything and run Tails off a DVD or USB, then connect the backup USB and copy your important files into the Tails persistent volume.