Even if he changes the password the Administrator can "remove" this password and then access the account with no password.
If it is just files you want to hide, although I don't know why you would, I would just download some kind of encryption tool and encrypt the files you want to keep secret.
You could also keep secret files on a USB drive. You can encrypt the entire USB drive's file system so it takes a password to access the drive in the first place. This is what I use at work to keep server passwords secure.