Is there a "super-admin" account in Win 7 or SBS?
The SYSTEM account on the domain controller has more privileges than even a member of Administrators. However, you are likely to cause damage if you start changing things that only SYSTEM has access to (they are protected from you for a reason).
(For completeness, though I definitely don't recommend it, you can run programs as SYSTEM by using
Other than that, there are no accounts more privileged than a member of Domain Admins. Members of Domain Admins are Administrators over the domain controllers (your SBS machine), and therefore can modify any user account and push any group policy out to any member computer.
As Stephen said, the System account does have different and in some cases elevated privileges above those of the various Administrator accounts, but this is not an account to play with, it's used by the OS and various services.
In a Workgroup the Local Administrator account has the highest privileges and in a Domain, a member of Domain Admins has the highest privileges