"Microsoft is urging users running Windows 8.1 to update their systems soon, since the company plans to drop patch support for early versions of the operating system.

Redmond said that after May it would only issue updates for systems running the Windows 8.1 Update release and later. The move means that Windows 8.1 systems which have not installed the "Update" package will receive "not applicable" notifications when attempting to install future system updates."

http://www.theregister.co.uk/2014/04/14/microsoft_version_of_windows_you_probably_havent_upgraded_to_yet_is_already_obsolete/