The upcoming retirement of which operating system is being called a “hacker heaven”?

As Microsoft discontinues support and updates for Windows XP on April 8, 2014, a flurry of attacks is expected to hit systems still running that operating system as hackers sit on vulnerabilities already discovered but not yet released. The last time Microsoft discontinued support for an operating system was July 2010, when it pulled support for Windows 2000, but 2000 never became very popular, while XP still runs on about one-third of personal computers worldwide.