It has nothing to do with the codecs.
You must set your TV to primary display, or it will show everything but the video.:
1) Right click on desktop and goto properties
2) Click the Settings tab, followed by Advanced
3) Now the ATI window should pop up
4) Goto Displays
5) You should see four boxes. One with Monitor, other with TV.
6) Click the first box under TV (near the bottom) (i think it is either a square or a circle).
7) Click the second box under Monitor
Now your TV will be on at the same time as your Monitor, however the TV will be primary. So you will be able to see the video on your TV and not the monitor. Hope u understand, if not, I can clarify it further. Good luck.
P.S.: You can display the video on both at the same time, however I wouldn`t recommend it, as it is very demanding for the GPU.