Valve has announced that steam will stop working on pc's still running XP and Vista on the 1st of January 2019.
"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 reason for valve dropping support for those operating systems is that the steam client is based on an embedded version of google chrome and chrome doesnt support xp or vista anymore. On top of that Microsoft also doesnt support XP or Vista with any form of security updates anymore and to buy stuff on steam you need to enter your card details and on xp and vista that is a big risk.
The new Steam Chat client is already not supported on XP and Vista.