If for some reason you’re still running on operating systems well over a decade old, you soon won’t be able to play games on Steam. Valve announced that starting on January 1, 2019 the Steam client will cease to support Windows XP and Windows Vista.
“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. 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.”
The reasoning behind this change is that new features on Steam now rely on an embedded version of Google Chrome that no longer functions on older versions of Windows. On top of that, Valve is continually updating Steam and states that future versions will require Windows feature and security updates that are only present in Windows 7 and above.
Windows XP and Vista will continue to run Steam games until the end of the year, but you may find some other features limited in functionality. The Steam chat client that is currently in beta will not be available to you if you’re using an older OS.