January 1st, 2019 is the day when Steam officially dropped Windows XP and Vista support, but it was still working and including updates, but suddenly on January 31th Steam automatically updated and resulted in an error “Failed to load steamui.dll”
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But it doesn’t stop us from using Steam as Valve said! The last working build of Steam on Windows XP/Vista is the January 4th build, But someone archived an earlier build of it from November 26th, that still works too. First of all download the .zip file including config.vdf. It has a steam.cfg file with settings to prevent further auto updates. When you downloaded it place everything in the C:\Program Files\Steam, when you finish copying all files place the config.vdf file into C:\Program Files\Steam\config. You will have to re-login to your Steam account.

For Steam users with the last working January 4th build!
To prevent automatic updates that update to broken builds download steam.cfg and place in C:\Program Files\Steam.
Now as you can see Steam works fine on XP without issues.