Other possibilities + solutions:
-- Right-click any video, click "Settings" and uncheck "Enable Hardware Acceleration".
-- Disable any Adblocking software, play a video; then you can re-enable the Adblock.
-- There may be another Player set as 'default' (like Real Player or Mozilla Shumway).
If that's the case, disable/uninstall it, and when you'd like it back, simply reinstall it.
-- Disable your AV utility while on YouTube, since you can't get a virus from YouTube.
-- You may unknowingly be using ''HTML5'' rather than "Flash''. To be sure, click here
http://www.youtube.com/html5 and opt out.
:-)