How to auto lock your computer
This FAQ will show you how to set your computer to lock automatically after a predetermined amount of time.
Type
Click Start and type: change screen saver and click on the “Best Match” result.
Set
The Screen Saver Setting windows will come up.
Set your screen saver to something of your choice from the dropdown list.
Tick the “On resume, display logon screen” box and set the “Wait” time to however many minutes you want and click OK.
As per MacIntyre's 'Our Safe Use of Computers and the Internet Guidance', which states “…the computer must be set to log off automatically after an appropriate period of inactivity (normally a maximum of 5 minutes).”, it's recommended to set the above to 5 minutes. The guidance states log out, but this will close any open files without saving them. By locking the computer, these files remain open, but the computer has still been secured.
Done
That's it.
From now on, when your computer has been left unused for 5 minutes, the screen saver will initiate and the computer will need your password to log back in again.