To install Windows 7 from USB you need a boootable USB. To do that:
1. Download the free installation file ISO using links in the email containing your recycled product key.
2. Download Rufus from here: https://github.com/pbatard/rufus/releases/download/v3.9/rufus-3.9p.exe
3. Use Rufus to create a bootable installation USB.
If you need assistance with Rufus please contact us.