it depends on what you want from your computer, what you want to do, how much money you have.
xp pros: less hardware requirements, faster with the same hardware, stable in 32 and 64bit versions, better array of software, more compatible with older and newer hardware
xp cons: highly targeted by viruses (there are others, but this is enough for the average user)
vista pros: less targeted by viruses, 64bit version is stable and fast
vista cons: slower on same hardware, less software available, less compatible with older and newer hardware, expensive, too much eye-candy
drop windows like the bad habit that it is, use ubuntu (it's the best linux for beginners)