Vote 1. XP
You may be able to get most things going on Vista, but I don't know anyone that has Images Plus working on it yet. Maybe there was a secret I was missing out on. Besides Vista really needs a gig of ram to work even moderately well whereas XP will get by with 512 meg. That extra 512 will make quite a difference with photoshop and other memory intensive programs (registax, Deep Sky Stacker
) They will operate without the extra ram but boy doesn't it make a difference when you have it.