SteamLink is an effort to run Half-Life: Uplink, Half-Life's single-player demo campaign from 1998, in Steam as easily as
possible, while leaving it as true to the original demo as possible. No in-game
content has been modified, and the "loading" and menu backgrounds are taken directly from the original Uplink, as opposed
to the generic Half-Life screens in Steam.
Simply download and run the installer. The installer will determine your Steam installation directory, and place files in the necessary directory. After the installer finishes, restart Steam, and "Half-Life: Uplink" should appear as a "Third party mod". In addition, a "SteamLink" Start Menu group will be created for direct access to Half-Life: Uplink.
Of course, the entire point of SteamLink is to play Half-Life: Uplink from within Steam, which means using the current Half-Life GoldSrc engine. The major benefit is better current hardware/Windows support. The downside is, for whatever reason, Valve decided to remove strafe leaning from GoldSrc a long time ago. You win some, you lose some.