Hi Kai,
Yes, you'll have to purchase a Windows 7 key. How about following these steps below, then repeat the last steps I've provided & see if you can get into the Windows Recovery Environment (WRE).
- Unplug everything from all USB ports, including your installation media.
- Go back to BIOS by repeatedly pressing the ESC key > as soon as you see the "Startup Menu", press F10 to enter the BIOS Setup.
- When in BIOS, navigate to "System Configuration > Boot Options" & ensure that the Hard drive is on top of the Boot Order lists for both Legacy & UEFI > press F10 to Save & Exit.
Now, this is the only last resort to get to the Windows Recovery Environment (WRE) to repair your Windows without any USB/CD.
If the boot error is persistent (with or without the USB installation media), that means both your USB & Hard drive are having issues. If that's the case, then I'd recommend purchasing a new Hard disk too.
I hope this helps. Let me know how you go. Thank you!
Sincerely,
Paul A.
Independent Advisor