Traditional TVs blur the picture, where HDTVs have a much sharper picture, so you notice more of the defects in the picture.
Same thing will happen when you jump to a higher-than-native resolution, such as doing 1080i conversion with your DVD player. You'll notice a lot more detail, but solid colors are a lot fuzzier as well.
But the real question is, why are you playing Halo 2?
