1) Make sure your antivirus is completely disabled.
2) Extract the compressed files using WinRAR and run the ISO file obtained after extraction using Daemon Tools.
3) Install the game.
4) Replace the files in the CODEX folder with the files in the installed game directory.
5) Enjoy playing the game.
Installation instructions for the FitGirl version:
1) Disable your antivirus.
2) Extract the compressed files using WinRAR.
3) After installation, run the game smoothly.