Go to the nVidia applet and go to the full screen video item:
(You can see it on the left on this image http://sparrowhawk.no-ip.info/Downloads/image001.JPG
, though this isn't what this picture refers to)
And select the fullscreen video device to be yer TV. Alternatively you could go to Windows Media Player, Tools menu, Options, Performance, Advanced and set check the Enable VMR. This should affect most of yer video playback. However some apps like PowerDVD don't use this setting, so you'll have to use the full screen video option. Other apps like Zoom Player can be configured to use VMR.
In any case if you like the fullscreen video mirroring option, there's no problem. There are issues with the sides of images on the video overlay being somewhat cropped on the TV out. Dunno if you'll notice it though. It tends to affect the TV out in anamorphic and 4:3 full screen clips.