How to save an animation file having .blend extension as a .avi file in blender2.42?

I have created a simple animation of a moving cube, a cube moving from left to right and have saved it as a .blend file. I want to save it as a .avi file. I typed /render/filename.avi in the first textbox of Output Panel, in the second text box i kept // and in the third textbox also i kept // . Then i pressed Enter. But it is not working. Plz guide me about how to save it as a .avi file.
Thanking you.
Sikha Devi

After filling in those details you need to press this big “ANIM” button in the render buttons.