Valve is going to stop supporting certain versions of Windows. On January 1, 2019, Windows XP and Windows Vista will not be supported. If Steam users with one of those two systems want to continue playing games after that date, they will need to upgrade to a system running Windows 7, 8, 8.1 or 10.
In addition, leading up to the Windows XP and Windows Vista support shutdown, Steam performance will be inhibited on computers with those operating systems. As an example, the new Steam Chat will not work on clients on either OS.
Here is Valve’s official explanation detailing why Windows XP and Windows Vista will no longer be supported.
“The newest features in Steam rely on an embedded version of Google Chrome, which no longer functions on older versions of Windows. In addition, future versions of Steam will require Windows feature and security updates only present in Windows 7 and above.”
Steam is available for Windows, Mac and Linux PCs.