1080i tells you that channel is picking up an HD signal, you can't/don't want to change that.
OK, the problem is for regular tv you need to be watching in 4x3 and in HD you watch in 16x9. There should be a setting on your TV that you can select which will distinguish them for you, what will happen is when you are watching regular non-HD tv the picture will stretch to fit the screen and at first you will hate this, but you will get used to it.
On HD stations you will get a picture across the whole screen ONLY when they are broadcasting in HD, on an HD station when the program is not broadcast in HD you will get the grey bars on the right and left side of the TV, again, nothing you can do about it.
Look through the setup menus and try to find something that let's you adjust between aspect ratios, or even on that give you options like Widescreen/Superwide/4x3/etc.... make sure this is set to widescreen.
HDTV sets have their ups and downs at first, you will get used to all of them and love it after a week.