(YouTube says) "Hello, you either have JavaScript turned off or an old version of Macromedia's Flash Player. Click here to get the latest flash player. "
But I have JavaScript on, and have the latest flash player. Despite that, I clicked again, receiving the message in Adobe's site: "Macromedia Flash Player was successful installed". Still nothing works....
Bottom line: You need to download and install the latest *full* flash player. I got mine from
http://www.softwarepatch.com/internet/flash.html
I'm not sure if this is YouTube's problem (for not recognizing the partial install) or Macromedia's (for not doing a proper partial install), but I had the same problem and now it works great.