If you need to leave your computer unattended, you should lock access to it by pressing [CTRL+L], which will log you off and bring up the log-on screen.   Anybody accessing your computer will need to enter their user name and password to proceed.   You can also set your computer to automatically lock access to your screen after the computer has not been used for a period of time, by using password protected screen savers (see your Microsoft Windows help manual).