This can be accomplished by auditing changes on the registry key corresponding to the Administrator account. See Q173939

Lost the administrator password? See Recover Lost NT Administrator Password for do-it-yourself, freeware, and commercial solutions.

A must have for NT administrators in corporate or governmental organizations or anyone being audited by a large outside audit firm.
It is not a secrets type guide but it has excellent sound advice and its used by PriceWaterhouse's auditors as a guide.