There have been quite a few recent posts about this error.
See this Wiki article - http://answers.microsoft.com/en-us/protect/wiki/protect\_defender-protect\_start/windows-defender-error-code-0x80073afc/29365ebc-5986-4408-a728-0600f41991b3?tm=1435026367942
Nothing definitive as to a cause or a solution, but some users have reported that a System Restore resolved the problem. Others reported that it did not help.
Alternatively, though it may be painful. you may wish to consider --
Restore, reset, refresh Windows 8: http://windows.microsoft.com/en-US/windows-8/restore-refresh-reset-pc -- this may actually be the quickest solution if you don't have lots of installed programs to reinstall.
-steve