Half-Life: Decay is the the third expansion pack for the award-winning game, Half-Life. It was included as part of the PS2 version of Half-Life. Decay returns to the setting and timeline of the original story, albeit portraying the story from the viewpoint of a different set of protagonists: two scientists working in the Black Mesa Research Facility. Decay is a cooperative multiplayer game, designed to be played by two people working together to pass through the game's levels.
This is one of the two demos released for Half-Life. The first one, called Day One, was just shipped with certain video cards and never released to the public. It contained the first day of Half-Life's story, from "Black Mesa Inbound" to "We've got Hostiles". Half-Life Uplink was the official demo released in 1999, and it contained a few unique levels that didn't appear in Half-Life. This is the original installer, it may have some problems with newer OSs and it doesn't have some features of the Steam version like 32-bit color mode. This installer doesn't require Half-Life (Original or Steam version).