Try this
Videos on YouTube are streamed through an Adobe Flash player. For the best viewing experience, we suggest you install the latest version of Adobe Flash after removing any old versions you've installed.
Please completely uninstall Flash before reinstalling the latest version.
To uninstall Flash, please follow Adobe's instructions, listed here:
http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=tn_14157
Once you're done, restart your computer and follow Adobe's instructions for reinstalling Flash:
http://www.adobe.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash
If the instructions above don't resolve the issue please try the following:
1. Temporarily disable popup blocking software installed on your computer to test.
2. Allow YouTube.com as a trusted site in your Firewall software.
3. Ensure other applications like Quicktime, iTunes, Real Player, or Windows Media Player aren't set as the default streaming application, as this might affect the video player.
Windows Vista users:
If the flash uninstall reinstall does not work this maybe due to a Microsoft Windows Vista permissions issue.
Please use the Windows button at the bottom left then the search function:
1. Go to: C:\Windows\System32\Macromed\Flash\
2. Right-click on the following file: FlashUtil9b.exe or FlashUtil9e.exe
3. Select "Run as Administrator" and install the update.
4. Restart your computer.
