Ok, it's very simple: under XP, but I think it will work under Win7 too, install CyberLink TrueTheater Enhancer. Then using Gspot, find in the list of codecs the CyberLink TTH h264 one, open its propery page, set on DXVA... Now hardware video acceleration for ALL youtube videos is working! Click on the blue tag that now appear into the right up corner of youtube videos for activate the mode! (Ignore TrueTheater setting, they don't work with DXVA). If someone find a way to hide the seek bar in fullscreen mode... It will be perfect!