I think Windows RT/Windows 8 tablets have proper user accounts. Hopefully they will come with a guest account as well. Not sure if the new apps know how to deal with multiple users and their data.
Windows really has the potential to shine in this department. Not quite a killer feature but it's definitely something they're capable of getting right.
It will only shine if the lag isn't interminable. Lag kills UX.
Not only that, but the user accounts can optionally live "in the cloud", such that a user - who would otherwise be a guest - can use another's device and keep their preferences by having them synced up automatically.