After installation, open the "CODEX" folder inside the mounted ISO, copy all its contents, and paste them into your main game installation directory, overwriting existing files.
Use a tool like WinRAR or 7-Zip . Right-click "part1.rar" and select "Extract Here." The software will automatically pull data from the subsequent parts.
Once extracted, you will typically find an .iso file. Use a virtual drive tool or right-click to mount it in Windows.
Execute setup.exe from the mounted drive to install the game.