I had a similar problem with my htpc and an nvidia 6200 using component out.
I had to go into the control panel and click the force tv switch and play with the gamma and color features to get it to work..
Took several hours but the time was well worth the effort.
Make sure you have an hd vid and a poor quality vid to work with!
It's always a hit and miss with this new technology, especially when integrating your pc with tvs.
hope you get it right...