Many security incidents (including a few that I’ve seen) result from an office worker leaving their workstation unlocked while unattended. One case I can discuss involves someone who sent a scathing e-mail to everyone in the organization from another employee’s workstation while that employee was away in a meeting.
In Windows XP and Vista, you can lock your workstation by holding down the “Windows” key and then pressing L. For Windows 2000, or for keyboards lacking the “Windows” key, press Ctrl-Alt-Del, then K.
The “Windows” key is near the Shift and Alt keys and looks like this:
Peter H. Gregory 
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