Play Half-Life with friends! Players fight computer-controlled monsters through the original Half-Life storyline or hundreds of available co-operative scenarios. Also includes a plethora of new weapons, enemies, visuals and gameplay systems. Customization features allow each map to use its own unique weapons and enemies.
Put the file in your HL folder Example:(C\Program Files x86\Steam\Steamapps\common\Half Life) Not sure if this works since the 'Steampipe update', whatever that is.