I'm assuming you have a regular 4:3 screen then.
Do you have this problem when your just playing your game OR all the time such as watching tv?
The only thing two things I can think of is:
- The menu on your tv.
- Setting the xbox 360 settings to the 4:3 screen.
You should be able to stretch the screen via buttons on the tv. You can make it wider or shorter or taller etc. Or it might be in your tv menu area. See if that helps.
Now if that didn't work I think you can change the xbox 360 screen size to 4:3.
http://techtips.salon.com/change-xbox-360-display-settings-4597.html