From my experience, you have to make separate substyles for XP and Vista/Windows 7. I find that the start menu has to be significantly bigger for Vista and 7 than for XP. And also, the width of the right and left panels in the start menu have to be different as well. Also, the start menu logoff images are different in each OS.
What I do is make the XP skin in SKS 5, then import it into SKS 7, make a new substyle for Windows 7, and then make the changes/additions in that substyle. I no longer have Vista, so I don't make a separate substyle for it, but will skin all the Vista elements, so that someone who has Vista, should, theoretically, have the Vista elements show.