Hello and welcome to SevenForums.
The account named "Administrator" is a special built in account that is disabled by default.
You should have been prompted to enter an Account Name during the install.
That is the account you should use to log in.
If you named this account "Administrator" during the install, I'm not sure what would happen, or if that's even possible.
Here's a SF tutorial about the "Administrator" account. Built-in Administrator Account - Enable or Disable
hope this helps,