Here is a comprehensive guide on how to do this.
Alex didn’t just copy the Windows 7 ISO file to a USB—that wouldn’t work. Instead, he used a free tool called Rufus (he downloaded it on a friend’s working PC). how to boot from usb windows 7
Booting from a USB drive to install Windows 7 requires two distinct phases: first, you must configure the computer to look for the USB drive, and second, you actually perform the installation. Here is a comprehensive guide on how to do this
Before you can boot, your USB must contain the Windows 7 installation files and a master boot record. You cannot simply copy-paste an ISO file onto the drive. Use a tool like the Windows USB/DVD Download Tool or Rufus to prepare the drive. 2. Access the Boot Menu (The Fast Method) Booting from a USB drive to install Windows
To force the computer to read the USB instead of the hard drive, you must access the pre-startup menu. How Can I Create Windows 7 Bootable USB Free Now?