Sounds like a problem with your Display settings on your Freeview box, you'll need to find the menu that adjusts the picture settings depending on the Aspect Ratio of your TV...
Usually the options are:
4:3 Cutout ( This is for 4:3 Tv's that dont want the black bars at the top and bottom when watching a show, you do however miss some of the picture from the left and right of the image)
4:3 Letterbox (This again is for 4:3 Tv's but this will show the full picutre putting black bars if needed for 'widescreen' pictures)
16:9 (or Widescreen) This is for widescreen tv's and should adjust the settings to show the full image for widescreen broadcasts or have black bars left and right for standard 4:3 broadcasts.
Sometimes you'll have 1:21:1 (or something similar) this is Anamorphic Widescreen often used on DVD films.. mainly used on Widescreen tv's, tends to make the picture fit the screen better.