You can change Administrator password from guest account without using any programs. It's easy and simple:
1.
Start-> Run, type lusrmgr.msc and hit Enter.
2.
You will see window with two folders: -Users and Groups.
3.
--Go in the Users folder,
--find the Administrator,
--right click-> Set Password and now you can change the password without knowing the old one.
4.
Restart your computer...
Enjoy it.... :)