First, uninstall Flash by using the Flash Uninstaller at the link below..
http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=tn_14157

After that's done, then download the appropriate manual Flash player installer from the link below. If running two browsers, you may need both installers.

Flash Player for IE 32& 64 bit:
http://download.macromedia.com/pub/flashplayer/current/support/install_flash_player_ax.exe

Flash Player for Non-IE browser (Opera, Firefox etc):
http://download.macromedia.com/pub/flashplayer/current/support/install_flash_player.exe
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After Flash is installed correctly, then please change the Flash settings like this:

So first, check if you can see the Flash animation (banner) without the system crashing at:
http://www.adobe.com/software/flash/about/

Don't worry, it's not a video, it shouldn't crash -- if it does, this won't fix your issue. If it works without crashing, right-click on the Flash animation (banner) and uncheck "Enable hardware acceleration" from the Settings panel. You should now be able to view the YouTube videos without crashing the computer.
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Next, if repairing Flash doesn't take care of the problem, instead of using WMP for playing your videos, try using a diffferent video player.. Download the free VLC player from the link below, install it, then set it up as the default player for all video types:

http://www.videolan.org/vlc/download-windows.html

Hope this helps.

Grif