There is no functional difference between the built-in Administrator account and an account granted administrative rights via membership of the Administrators group.
...other than the fact that UAC is disabled by default for the built-in administrator account. Also, some legacy software written by monkeys won't install on local or domain accounts with local administrator group rights, and require the built-in administrator to install.
(this being bad practice, as the best-practice methods teach you to rename, re-password and disable the administrator account, create new accounts for administration with admin rights, and create a new account called administrator with very few rights as a honey-pot).
