In Windows 7, when you want to lock your workstation or log off, you click Start and then click the tiny little triangle to the right of the Shutdown button and then click either Lock or Logoff as desired. But how many times have you (or your users) tried to do this and accidentally clicked the much larger Shutdown button instead of the tiny little triangle?
To prevent users (or yourself) from accidently shutting down your computer when you only want to lock it or logoff, use Group Policy to configure the setting named "Change Start Menu power button" so that Shutdown button is changed to either Lock or Logoff as desired. This policy setting is found under User Configuration, Policies, Administrative Templates, Start Menu and Taskbar.
Now when a user wants to shutdown her computer, she'll have to click Start and then click the tiny little triangle to the right of the Lock (or Logoff) button and then click Shutdown. This will reduce the chance of unintended shutdowns occuring.