• Open Run Dialog Box (Start > Run).
  • Write "secpol.msc" to open the Local Security Policy editor.
  • Navigate to the tree structure in the left pane, in Local Policies, expand it to Security Options.
  • In the right pane, there is a Policy Accounts: Rename administrator account, Double click over it.
  • Change the name of Admin account to the one you wish to have now.
  • Close the Local Security Policy window, Log off to save the changes.

Note that: this will not work on Windows 7 Home or Basic.