Decrypt Admin password using the grup.xml file
Depending on the settings of the "Group Policy Preferences" you might be able to elevate your own login.
A perfect HowTo can be found here.
You need below decrypt key, that MS publishes via the Internet. :-)
Prove of concept
And decrypt the local Administrator password using the public decrypt key from:
http://msdn.microsoft.com/en-us/library/2c15cbf0-f086-4c74-8b70-1f2fa45dd4be.aspx#endNote2%C3%A2%C2%80%C2%9DAfter discovering the password you "own" the machine. Microsoft have addressed this with MS14-025 - http://support.microsoft.com/kb/2962486 effectively removing the option to save passwords but it does not do anything for ones already set. MS has also released a new tool which might help with random passwords http://blogs.technet.com/b/askpfeplat/archive/2014/05/19/how-to-automate-changing-the-local-administrator-password.aspx
Have the local Administrator password change in very regular intervalls
See this TechNET articleSolution(s)
Reset Local Administrator Password Using A Different Random String On Each Computer And Recover The Passwords Securely
See this article: