Instructions
Download the Infernus Mod from Cnet Downloads.
Open the installer for the Infernus mod once you download it, then click "Next" on the installer screen.
Click the radio box next to "I Agree" to accept the terms of use.
Select the installation directory where "Grand Theft Auto: Vice City" is located. If you used the default installation, it will already be filled in for you by the creator of the mod. Click "Next" twice to begin the install.
Run the mod from the "Grand Theft Auto" directory before you open the game to use it.