GM, if you have MS Photo editor (I can't remember if it is a Windows accessory or an Office accessory) you can easily do it - just open the image and choose Image / Resize - I find it simplest to change units to "Pixels" and enter 640 as the width - as long as you don't have Allow Distortion it will automatically enter the other dimension for you.
I know it is fine for BMP and JPG - don't think I've tried it with anything else.
SD