Hiya ST - I use a Barbara Currie Power Yoga video which is ok, but I don't think any exercise video alone will make you lose weight. We have got a lady posting here who teaches aerobics, etc, and even she has to count the calories as well - even though she is incredibly active.
Have a wander round this bit of 3FC and you will see that we all do different sorts of exercise, but I think most of us do it to tone the muscles.