well there are mac operating systems. Mac os x. get a mac it rarely gets viruses. although they are a bit expensive it's still worth the money. they last a very long time
The operating systems you are referring to are similar, and I have both, but prefer XP. If you are speaking of different computer systems, there's MAC and Linux, both good in different ways than a PC.
xp and vista are operating systems; both pertaining to PC. The other computer is Mac; which is fast becoming as accessible as PC and besides is better for graphics applications. As far as xp and vista; vista is simply the newer of the two