1. If you're building a new computer, why is it already running Windows 8?
2. That backup is particular to the original hardware. Seeing all new hardware, it will choke
3. Licensing - depending on exactly what Windows 7 version you have, it might deactivate itself
4. 1 license, 1 PC. If you want to use the existing license on the new PC (if you can), you can't also use it on the old PC