It depends on which account they log on with, the same as with a Mac. They can also set an account to log in automatically, like they can on a Mac, if they want to, but it isn't necessary. It doesn't make any difference to the security, whether Windows displays the start screen or the desktop - users can manually choose to go to the desktop now, but many don't know that.

As I said, this is what appears AFTER they have logged on.