Well lets start with the cable box. Did you just upgrade to HD? I know when the comcast guy came out and setup our box he plugged it all in but he didnt set the BOX itself to output at 1080i. It was still set to 480i. Direct TV did the SAME thing with my grandparents-in-law's box. So make sure the box is set to output in high def. For the comcast boxes you usually have to turn the box off then press MENU and a white menu will come up and you can set the resolution there. Now another thing i want to know is if you do indeed have high-def programming? I get a lot of folks coming into my store expecting to see there programming in high-def just because they bought a high-def TV. Im not trying to insult your intelligence here but i figured i would ask.
Now with dvd movies there are two things. Dvds will never really be CLEAR on a high-def TV, they are 480 line movie being stretched to fit on a 1080 line screen. HDMI Upconverters and line doublers do a decent job at enhancing the quality but DVDs are what they are, a 480 line picture, so don't expect crystal clarity there, thats what they make blu-ray for. Second you might want to turn the automotion plus feature down to low or OFF while watching some DVDs. It can make them have weird artifacts or ghosting around the movement. I find that off to medium is best depending on the movie. Now if you get a blu-ray player and you set the player to output at 24p then go ahead and jack up the automotion to high.
Try these things, its so rare for a new TV to be defective and when they are they usually have multicolored lines splotches on them or they just flat out dont turn on. Im thinking your TV is probably fine but you may be expecting too much from dvds stretch on a 52 inch screen. And please let me know if you do indeed have the HD package from your provider, if you do, then check the settings on your box, if you dont have the HD package than you need to get it and that's why your picture is blurry.
One more thing, you said you have a non-hdmi box, are you using the green red and blue component cables then?