It is possible to change Adminstrator password in Win XP and Vista without knowing it. Read this tutorial and give it a try in your friends PC.
1. Right-click on the My Computer & select the option Manage.
2. On the left frame, double click on Computer Management (Local), then System Tools, then Local Users and Groups & lastly on Users.
3. You will see the list of users on the right frame.
4. Right click on Administrator and select the option Set Password.
5. A warning message will appear. Click on proceed.
6. The system will ask for New Password and Confirm Password.
7. Enter a password & click on OK. The password is changed.