I'm having the EXACT same issue! 1/2 of YouTube videos don't work. Hulu does NOT work at all! I'm running Mac OS X 10.9 on a 2.26 Duo. (sad i know).
I was running Chrome but when I checked my Activity Monitor it was using 987.23% of my processor. So I killed it and am now using Safari and FireFox. However now both of those browsers are just giving me a "plug-in failure".
I was just trying to watch http://www.hulu.com/watch/552963 and it started to load and play (audio and video) then went right back to "Plug-in Failure".
-Aaron from WeBuyPinball.com
There IS NO SUCH THING as an OS X virus... never has been.
Chrome is a HUGE resource hog. I ununstalled it and will never allow it on my Mini again.
Try doing a Clean install: How to perform a "clean install" of Flash Player in Mac OS X
The ONLY step I'd deviate from is locating the Flash Player plugin manually. Do a Spotlight search of your Mac HD for "Flash Player.plugin" and trash EVERYTHING that appears in the results. THEN follow the other steps to reinstall and ensure it's enabled in Safari.