I'm afraid you're almost certainly going to be out of luck (especially as i see Cannon stopped driver support for the S400 with XP)
Now there is one remote chance for an alternate solution.. and that's a Windows 7 capability called "virtualization" but it requires both 1) You run Windows 7 Professional and 2) Hardware support from your CPU.... Are you running a Windows 7 Home version? If so, we needn't check your CPU capabilities