There are two ways of locking a machine. One is to install a mechanical lock that has to be opened by a key. Most of the PCs come with this lock.
Under Windows NT, the Logon Process and the Security Subsystem jointly ensure access restrictions and protection from unauthorized access. The Windows NT Logon Process provides for mandatory logon to identify users. Each user must have an account and must supply a password to access that account. The logon name and password provided by the user is authenticated by the Security subsystem. After successful authentication, whenever the user accesses an object, the Security Reference Monitor ensures that the user has permission to access the object.