Hmmm.... sounds good, but let me tell you that making a WindowBlinds skin is difficult, you need too much images, things, writing, etc. If you have done a theme before or you know how to do it, do it. If its for xp, do it!. If it is an Vista Aero or Longhorn adaptation for Xp, please, DO IT!!!!!!!!!!!! YEAHHHHH!!!.
But if you haven't done themes before, I'll sugest you to make an normal "Visual Style". There are programs that do it easily, like StyleBuilder, or if you like free and difficult things, and if you want to do some coding, try the famous "Resource Hacker".
Anyway, let the mockuping begins!