Make sure that your antivirus software is completely disabled.
Extract the archive files using WinRAR and mount the ISO file obtained after extraction using Daemon Tools.
Install the game.
Place the files in the CODEX folder in the game installation directory.
Enjoy playing the game.
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Fitting instructions for the FitGirl version of the game:
1) During installation, make sure that your antivirus software is completely disabled, have enough space on your hard drive and RAM, and be prepared to spend a lot of time installing the compressed version. At the beginning of the installation, uncheck the additional options. The additional options are: Update DirectX, other languages other than English, and different versions of C++ for the game (if you haven't downloaded them yet). Please note that it is recommended to install all 32-bit and 64-bit versions of Microsoft Visual C++ from 2005 to 2015 on your system to avoid any problems. For this purpose, you can download the entire prerequisite package from here.
2) After completing the installation, run the game and enjoy it.