https://www.pcgamer.com/legendary-half-life-mod-sven-co-op-turns-20-years-old/

Half-Life, released at the end of 1998, did not have cooperative gameplay. That was unacceptable to Sven Viking, who on January 19th, 1999 released beta 0.8 of his mod Sven Co-op, a proof of concept multiplayer modification consisting of a single level of the campaign. Today, 20 years later, Build 3482406 of Sven Co-op is available. That’s exciting, unless, like me, it makes you feel the inevitable march of time and the looming of the grave all the more keenly.

The lovely update squashes a healthy multitude of bugs and rejiggers some of the various checkpoints, along with the campaign from They Hunger. It also adds a glorious thing: varied sounds for NPCs firing the MP5 submachine gun, one of the most grating repeated noises in all of video games history. The patch drops support for Windows XP and Windows Vista, operating systems which weren’t even in development when Half-Life and Sven Co-op were first released.