on the xbox av cable change the switch to normal mode (not hd) and just plug in the yellow, white, and red plugs. then go to the settings and change your display settings to letter box. that should help. some hd tv's (mainly the cheaper ones) just do that, the hd signal is in a pre determined format leingth x width that some hd tvs dont nesecarally support. I have a 58" hd rca that cuts off about an inch on each side, but my 56" pioneer plasma dont do this. this is a sales gimmic to make you think its a bigger tv, it is taller, but not as wide as it should be, for as tall as it is and the tv dont know what to do with the rest of the signal so it cuts it out.
did that make since? try running it in normal mode and see if that helps. you could allways buy a better tv LOL