Yes it is possible to change Adminstrator password both in Win XP , Vista and 7 without knowing it. If you somehow manage to login as an Administrator then you can change the password without knowing it.
Here’s the Step-By-Step instructions
1. Right-click on the “My Computer” icon on the desktop OR in the Start Menu.
2. Select the option “Manage”.
3. On the left pane expand the “Computer Management (Local) tree (if it is not already expanded)
4. Double click “Local Users and Groups” option From “System Tools” tree.
5. Click the “Users” option.
6. Now you will see the list of users on the right side pane, such as Administrator, Guest etc.
7. Right click the “Administrator” and select the option “Set Password” option.
8. Now you will see a warning message, Click on proceed.
9. Now the system asks you for “New Password” and “Confirm Password” .
10. After entering the password click on “OK”. The password is changed. That’s It!