If you’re still gaming on a PC running Windows XP or Windows Vista installed, first of all well done! However, if you want to continue gaming using Steam, it’s time to start thinking about upgrading. Valve is ending support for XP and Vista on Jan. 1, 2019.
The announcement was made on the Steam Support pages with Valve explaining that from next year the Steam Client won’t run on these old versions of Windows. The only way to fix the problem is by upgrading Windows. It means embracing Windows 10 or sourcing yourself a license for Windows 7, 8, or 8.1. The safest long term option is Windows 10, though.