Comments - achefdeadfrontier

To fix that you can start the game using the bootstrap parameters.

Click the Create a shortcut on your desktop to hl.exe
Click properties on the shortcut
and add
-game decay
That forces the hl engine to read the folder named decay (Half-Life: Decay folder)

No-Steam example:
Destiny: Drive:\Valve\hl.exe -nomaster -game decay
Start in: Drive:\Valve

Steam example:
Game should be installed in: Drive:\Steam\steamapps\common\half-life
Restart Steam and you should be able to see the game in your library.
In case you can't just do the same as for non-steam on Half-Life
Right click, Properties, Launch Options, -game decay

Glosary: Drive represents the partition were you have the game/steam installed, example. C: drive is Windows (Default)

Hope this helps