https://www.techradar.com/news/steam-games-will-stop-running-on-windows-xp-and-windows-vista-next-year

If you’re a PC gamer with a large library of Steam games, and are running Windows XP or Windows Vista, then you’ll want to upgrade to Windows 10 (or Windows 7 or Windows 8.1) pretty soon, as Valve has announced that Steam will no longer run on those older operating systems from January 1, 2019.

Valve announced the decision on the Steam website, where it confirmed that “the Steam Client will no longer run on those versions of Windows. In order to continue running Steam and any games or other products purchased through Steam, users will need to update to a more recent version of Windows.”