This is a frequent problem with Flash, which Youtube uses. Right click on the video. Under the far-left tab (tabs are found at the bottom), look for the box "Enable hardware acceleration" and uncheck it.
If there is a message stating that you can't change hardware acceleration, then you might try going into your display/graphics properties (right click on the desktop) and either lowering your resolution or your color quality (usually from 32 bit to 16 bit).