Red-dotted rectangle appears within camera view

I have routinely got a scene in camera view and rendered (using HDTV res at 50%) with output to a Mpeg file.
But recently I noticed that in a walk animation, a small red-dotted rectangle appears within the camera view.
The animation is only rendered in a sieries of images (Jpg or Png) AND ONLY WITHIN the small dotted rectangle - so most of the mesh (seen in the camera view) is missed.
If I choose output format Mpeg, it won’t render at all. An error is flagged: “Error writing frame”.
Would someone kindly explain
a) why the small red-dotted rectangle appears, so missing most of the animated mesh
b) why the Mpeg format is rejected
I’m using Blender 2.62 and Windows 7.

Screen image is attached below - it’s a figure walking in camera view. The small red rectangle is a bit difficult to see.

Help would muich appreciated.

The red box is a render border you have selected (Shift+B). Deselect the Border tickbox in the Render / Dimensions panel.

That’s a render region; it limits the area that needs to be processed – press shift+B and draw a box outside the camera view (in the shaded region) to make it go away.

Thank you, Richards and Benu, I unticked the ‘borders’ button and that did the trick. The red rectangle disappeared.
I don’t recall consciously ticking that button in the first place.
Grateful thanks.

FWIW, you don’t have to check the box, simply pressing Shift+B and dragging a box in the camera view turns it on. You may have accidentally pressed that key combo when trying to do selections (B) or hiding unselected (Shift+H), etc