That's really strange, XP should work without problem with eBox. Are you using our installer? Or are you using the packages in a distribution different to Hardy?
We will upload a new samba 1.2.4 package very soon that will add support for newer samba versions, and we have a samba 3.4.2 package in our 1.3 repository. Maybe you can try it once 1.2.4 is ready to see if that fixes your problem.