It doesn’t happen often, but more than it should. If you can’t play YouTube videos or from any other online video source, you have a problem.
Installing over the top of what is already there doesn’t work. You have to first uninstall..
Link here for the uninstaller.. http://kb2.adobe.com/cps/141/tn_14157.html
When you come to re-install AF, it may report that the install was not successful, but a little perseverance will get it back on track..