An interesting note on Vista is the claim that every Vista product disk has all versions of Vista on it.
Microsoft has a program called "Instant Upgrade" and if you don't like the Vista version you've got you can call them up with your credit card number and they'll unlock another version for you to like even less, all from your own one original disk.
Maybe the intent is to keep everybody unhappy, in hopes people will do a lot of version swapping.