Change Local Admin Password Through Group Policy

First enable windows 7 "Administrator" account because "Administrator" account is disabled by default in Windows 7.

Enable WINDOWS 7
Administrator Account with Group Policy (GPO)

Administrator account is disabled by default
on Windows 7.
Group Policy can be used to enable it.
Create a new GPO and
go to: Computer configuration -> Policies -> Windows Settings ->
Security Settings -> Security Options. On Right Pane click on Accounts:
Administrator account status and change the settings to
Enabled.

To enter a password
for the Local Administrator Account, Group Policy Preferences can be
used:

Go to Preferences -> Control Panel Settings -> and right
click on Local Users and Groups. Choose: New -> Local User. From Drop-down
menu on User name choose: Administrator (built-in).
Put the password and
confirm it.