Steam Support for Windows XP & Vista ends today

[kemical]

Support for two former operating systems, XP and Vista will no longer be in effect after Jan 1st 2019. This is due to embedded software within steam becoming incompatible with the old os's:

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 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.
For the remainder of 2018 Steam will continue to run and to launch games on Windows XP and Windows Vista, but other functionality in Steam will be somewhat limited. For example, new features such as the new Steam Chat will not be available. We encourage all users on these operating systems to upgrade to newer versions of Windows in order to have ongoing access to the latest features of Steam, and to ensure future access to all games and other Steam content.

Ref:
Windows XP and Windows Vista Support - Steam - Knowledge Base - Steam Support

 

[Yart]

So if you have a version of Steam that wasn't updated (pre-Chrome additions to the chat system), you can pop a file in the Steam folder called 'steam.cfg' containing the following:

BootStrapperInhibitAll=enable
BootStrapperForceSelfUpdate = disable
ForceOfflineMode=enable

Boot up steam, log in, then from the library window under the "Steam" drop down menu, "Go online."

Might take a couple tries, but when it works, boom! You're using Steam in 2019 complete with chat support in Windows XP.