DVDs are 480 lines of resolution, which is standard definition as far as signals go. The fact that you'd be getting a poorer result on the screen when compared to a HD signal is understandable.
The Roku might have a better display through the component ports if you adjust the output resolution in the menu. I know that component is not the destination in this situation, but to explain: in some cases, devices are designed to default to low resolutions. (for instance, a 720p TV might not accept a 1080p signal, but a 1080p TV would accept a 720p signal, and you could adjust the settings accordingly).
Some DVD players also have the option for 480i and 480p - if you can check and select 480p as your resolution.
I think these are SD-to-HD issues. But that said, you noted that this is recent. Without knowing what happened, I'd need to know what changed to offer any kind of solution.
Hope someone can help. I have a Samsung ln32b460b2d LCD television. Works great except that my AV inputs and my component inputs do not seem to be working correctly anymore. I have tried two different DVD players, two different connections, RCA and component and both result with a fuzzy sort of pixalated DVD display. I have my Direct TV and my Roku hooked up through HDMI and I get great sound and picture with both of those. I know that the issue does not lie with the cable or with the DVD units themselves because I have hooked my DVD units to another TV and the work fine. Just for troubleshooting purposes I hooked up my Roku to the inputs in question via RCA and component and I got the same grainy pixalated picture. I had my DVD player hooked to this unit before I moved recently and it worked fine, it seems like ever since Direct TV hooked up my new cable box I have had this issue. I seem to feel that it seems to be some sort of setting within the TV itself....its really driving me crazy!! ANY help would be greatly appreciated!! Thanks for your time!