Is there programs or web apps available to draw a draft of each layout from an app prior to developing it ?
I'm not looking for something that will generate the xml layout but just a simple tool to draw a sketch for each window of my app
You're talking about mockup and wire-framing tools.
There are quite a few of varying price and quality, here's a few:
I can't vouche for any of these, but they should provide what you're looking for, although pen and paper will probably outmatch them easily! ;)