I’ve been to many clients’ homes where the complaint was that a visitor (perhaps a friend or family member) has been given access to the computer and now it doesn’t work right. When I look at the user accounts that have been created, there is generally only one user and that is set up as an administrator. Not only that, the account is not password protected.
The best way to keep the little (or large) gremlins out of your computer is to enable the guest account and let your visitors use that. Next, password-protect your own account. If you have more than one person using the computer at home, create a second or third account as needed. Consider making those account limited accounts—this limits the “damage” that others can do! Oh—and resist the temptation to give someone else your password just to make them quit nagging you.
Oh—and another thing. Every computer has an administrator account. It is not usually visible. I find that most people have never assigned a password to that account. This means that someone can log on as the administrator and do anything that they want—change passwords, change system settings, and generally make your life miserable.