If you need an original, untouched x64 ISO for a legacy system, testing environment, or industrial application, you must use specialized alternative channels:
: Select the primary partition, click Format , and then click Next to begin copying files. Post-Installation Security Warnings
Built-in Windows Defender on Windows 8.1 no longer receives definition updates. You must seek out a third-party antivirus provider that explicitly still supports Windows 8.1.
If you have a paid subscription, you can log in to my.visualstudio.com to search for and download verified Windows 8.1 ISOs with official SHA-1 hashes.