As I thought, the problem was the bootable USB(s). They were all 4GB ones, so I used a 8GB one and made it bootable with Microsoft's Windows 7 USB/DVD Download Tool
and a free Windows 7 iso
After that, I installed the drivers from AMD and ASRock and upgraded my system to Windows 8.1. I was dumb—you should install the GPU driver (so the Win8 updater can run under the right resolution) and then install only the Win8 drivers. Anyway, it's working!