Choose your language and click "Repair your computer" (do not click "Install now"). Important Safety Warning
Go to Control Panel > System and Security > Backup and Restore. Click "Create a system image" on the left sidebar.
When prompted to create a repair disc at the end, Windows may allow you to point to a USB drive depending on your specific version/updates. Method 3: Download a Windows 7 ISO (The Professional Way)
Official Source: Historically, Microsoft allowed downloads via a product key, but this service is largely discontinued for Windows 7.
If your PC won't boot and you don't have a disc, you'll need to download a Windows 7 ISO file from a working computer.