Your choice. XP is a good solid OS, but Vista is perfectly fine and usable as well. The drivers are the same for the most part but if you are giving an option, I hope your are wise enough to figure out which to go with.
Windows 7 probably wont be an option, but you should probably look at their website and the drivers they provide there for it. If they have Windows 7 drives then you should be fine. IF they don't, then it is a crapshoot at best.