Why *must* the Administrator icon appear? The Administrator account is only intended for emergency purposes. If you need administrative privileges, you should be creating a separate account for those purposes. That's why it's not present by default but still accessible w/ ctrl+alt+del.
Administrator is the default account that I used before
Not a good idea. You should always create a separate account w/ administrative privileges. If you screw up something within the Administrator account, then you leave yourself with no other working option! Leave the Administrator account alone (or at most give it a secure password) and use a separate account. Again, that's why it disappears. It was never intended to be used day to day.