Since I got Comcast DVR, all non-HD channels show in 4:3 or 16:9 with a black border on all sides only and the option on my Samsung TV to change the picture size to "Screen Fit" is now grayed out. Before I had the DVR, I could watch non-HD channels in "screen fit" size. I tried unplugging, reversing the coax, etc., but nothing has worked. Perhaps the only solution is to give up DVR?
Make sure when you want to watch your cable set top box to select the proper TV input that the component cable is matched to. In otherwords, if your component cable is connected to video 3 on the TV, then you need to select vidoe 3 on the TV's remote. Do not watch your cable box using the TV's tuner on channel 3. Watching with channel 3 will never get you a real hi def picture, and it will never be wide screen. You must use your component input.
Next step is to properly set your set top video to match your TV's capability:
On the Set top box remote turn off the set top box, and within 2 seconds, press the "menu" button.
There will be a menu come up on the screen, and you need to make sure 16:9 is selected for aspect ratio. Below that is a selection to set the output to 1081i, make sure that is enabled. You can also choose if you want your standard def channels to be in 4:3 or stretched.
The menu will vary if it is a Motorola, Scientific Atlanta / Cicso or Pace RNG110.
Please ask if you need more assistance, and if you do, post the model of your set top box.
Again, make sure you are not watching with your TV tuner on channel 3.