First, I would ask why you chose XP Embedded. XP has enough problems, I could only imagine how bad they are on this version.
If it were me, I would put my faith in Embedded Linux. There is a pretty good support at NI for Linux, and embedded Linux works great, I saw an example of a system on a PC104 with Linux (not embedded version.) If you want to know more about that, get a copy of Gary Johnson's book "Power Programming, 3e"
If you are going to stick to your guns and go ahead with Windows XP Embedded, then I would have to believe that you are pretty much on your own here.
Contact NI. They should be able help you in this venture. They are great about helping with projects like this. We had their help when we built the first MFC DLL in LabVI
EW.
Good luck.