For privacy and security sake, it is a good idea to always Lock or Loggoff of your computer when you are going away from your desk.
Generally speaking, if you are going away for just a few minutes, simply locking the computer is recommended. To do this, just hold the Windows key on the keyboard and hit the letter L. The Windows key is located two keys to the left of your spacebar.
Locking will put your computer at the log in protection screen where you will need to hit Ctrl-Alt-Del and then type in your username and password.
Read the message on this screen before you hit Ctrl-Alt-Del - it will say that only you or an administrator can log in to the computer because its locked. This prevents other users from logging in to your computer using there account.
To Logoff - click on StartShut Down and select Logoff from the drop down list. Logging off instead of Locking will preform a 'neater' security process which will Synchronize your files if you have that option selected and will prepare your computer to be logged in by others if needed.
To Logoff - click on Start>
If you share your computer with others regularly, it's best to get in the habit of logging off instead of just locking. Also it's a good idea to log off instead of locking when you go home for the day. Of course you can also shut down your pc at the end of the day for security sake. Your company will likely have a policy or preference as to whether you should turn your computer off at the end of your day.