Offline NT Password Editor
is a freeware utility that can change the password of the Administrator account. Useful when having lost the password for a machine or just gotten ones hands on a machine where no one knows the password.
Other free utilities that can help you:
There also exist utilities one can pay for:
There also exists tools for extracting the password hashes (the calculated value based on the password) from the local or a remote system, and tools for turning the calculated value into a password using bruteforce and dictionaries.
Related Reset Administrator password