Ensure your Windows 8/8.1 key is ready. If you are upgrading from Windows 8, you may need to install 8.1 as a "free update" through the setup environment [10, 25]. using NTLite?
If you need this OS for compatibility, your best bet is to create your own ISO using the WSUS Offline method. If you must download a pre-made file, verify it against checksums provided by trustworthy sources like the "Heidoc ISO Downloader" (which unfortunately no longer supports 8.1 actively) or archived MSDN images. windows 8.1 fully updated iso