DSLR Shutter is only a snap program, it will take a pic via a bulb cable but will not display the image. Various programs I have used are MaxIm DL / MakIm DSLR and another program called Focus assist written specifically to focus the Canon 400D and uses the cameras USB cable. So I don't think it will work in Vista yet as the 400D is unsupported in Vista.
http://www.xmission.com/~jstanley/focusassist/
Give it a try, works okay but its not automatic.