As of April 8, 2014, Oracle is no longer supporting the use of Java on Windows XP. Java 7 can still be installed on Windows XP, and Java 7 updates installed on Windows XP will probably work as expected, but Oracle says you’re on your own if bad things happen. Java 8 will refuse to install on Windows XP.

Recommendation: if you still have computers running Windows XP, stop using Java on those computers.

Update 2014Jul18: Oracle recently posted a clarification, saying that Java issues affecting only Windows XP will not be addressed with updates. Java issues affecting Windows XP as well as other versions of Windows will get updates, and those updates will work as expected on Windows XP.

Edit 2014Jul18: fixed two typos in the first paragraph.