https://motherboard.vice.com/en_us/article/d3kkmw/steam-on-windows-xp-valve-new-features

Windows XP and Vista users have six months to upgrade their operating systems or get the hell off of Steam.

“Steam will officially stop supporting the Windows XP and Windows Vista operating systems,” Valve, the company that operates Steam, said in a post to its XP and Vista support community. “This means that after that date 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.”