As far as I understand, you want to read their chat logs without their notice. "DisplayWizardFirstStart", disable this option (you can learn how to do here
Messenger Plus! Live - Registry Settings), because if people who firstly signin on your computer read the wizard, they will notice that Messenger Plus! can save their chats. Also not showing Messenger Plus! buttons on Windows Live Messenger windows may prevent them to notice that Messenger Plus! is installed on the computer.
If they already signed in on the computer, you must add the suitable keys for chat logging to the registry key of each person. If the information above is not enough, I'll write more about how to add keys to the registry.
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