YouTube - Black Screen with Windows 8

Hi,

I have a new computer with W8 and I cannot play videos on youtube. Most of the time all I have is a blank, black screen and the video does not start, either on Chrome, IE 10 or Firefox. It plays the videos 1 out of 10 times. It was fine this morning, I turned off the computer and now it is not working anymore. I can play videos on other sites such as Dailymotion.

JavaScript is enabled.
I have the latest Flash Player version installed.
Antivirus is Norton.

Thank you for your help.

Cedric