If any of your computers are shared among family members, make separate user accounts for each user. Put passwords on each account and do not share your passwords. Make only one account an “administrator” (you – since you are reading this!) and make all other users a “Limited account”. Turn off the Guest account.
When a family member is done with the computer (even for a minute), get everyone into the habit of locking the screen, which requires a password to unlock. This will prevent one person from using another person’s computer account. Click here for instructions.