Microsoft has issued a security update to deal with a vulnerability which could have lead to a global virus outbreak. They regard this as so serious that they've issued it even for XP computers which they stopped supporting in 2014.
https://www.bbc.co.uk/news/technology-48295227
The other Windows operating systems which could be affected where they've issued an update are Windows 2003, Windows 7 (I think that was included in the update they did yesterday), and Windows Server 2008.
Apparently 3.75% of desktop computers in use have Windows XP !