There have been countless posts, here on AB, about problems viewing the content of sites which use Flash video (such as Youtube, DailyMotion, etc). The problem seems to come about when Adobe Flash Player is installed but not working properly. You can't watch the videos (because the player isn't working) and you can't freshly install the player because your PC thinks that it's already there.
The way to break this vicious circle is to completely get rid of Adobe Flash Player and then you'll be able to reinstall it. So, go to Add/Remove Programs and remove all references to 'Adobe Flash Player'. Then download the player from here:
http://www.adobe.com/go/getflashplayer
Most (but, regrettably, not all) previous posters have found that fixes the problem. Let's hope that you're in the majority ;-)
Chris