Setting default (preferred) decoder did not help in my case. Try to remove all other codecs. Then add the other codecs you need. (divX or whatever you need more)
Well I removed all the other codecs, played a DVD but still the nVidia codecs icon was not appearing. However, when I also removed the nvidia codecs I got an error so I assume that I'm actually using the nvidia codecs.
Same thing with Windows Media Player, with Zoomplayer the icon is appearing. Strange...
Anyhow, my HDTV clips are playing much better now and my CPU load is low so I guess I'm ok.