First, uninstall Flash by using the Flash Uninstaller at the link below..

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:

Flash Player for Non-IE browser (Opera, Firefox etc):

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:

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.

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:

Hope this helps.