https://www.pcgamer.com/steam-will-drop-windows-xp-and-vista-support-at-the-end-of-the-year/

Windows XP was a great operating system, but it's also 17 years old, and its best days are long, long behind it. And on January 1, 2019, Valve will finally pull the plug, and send the old boy off to OS heaven. 

"Starting on January 1 2019, Steam will officially stop supporting the Windows XP and Windows Vista operating systems. This means that after that date the Steam Client will no longer run on those versions of Windows," Valve announced today. "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."