Microsoft publishes official checksums for each ISO. Compare the downloaded file’s hash using Get-FileHash in PowerShell (Windows) or sha1sum (Linux).
After creating a bootable USB drive:
Right-click the ISO → Properties → Digital Signatures tab. A valid signature should show “Microsoft Corporation.” windows 8.1 iso file download
After installation, you'll need to activate Windows 8.1 using a valid product key. Microsoft publishes official checksums for each ISO