If you are using TMT you have to not only have hardware acceleration checked in the stand alone player from the desktop, but also in the one launched from inside WMC if you are using TMT from inside WMC. They use separate executable files that don’t share settings. Right click while playing a video and go to the settings menu. This bit me when I was first trying to get it running, everything was fine in the stand alone player but was choppy with 100% CPU usage when TMT was launched from inside WMC.
