You can log a chat either two ways through MSN or through plus!.
The MSN way : Tools>Options>Messages
and at the bottom there should be Message History check the option "Automatically keep a histroy of my conversations" and choose where to save the logs.
The Plus! way : Plus!>Preferences>Logging check the option "Automatically log all my chats by default" and choose where to save the logs.
The plus! way is best as you have more options like encrypting the chats with passwords and when to start logging after so many minutes. Hope this helps you out.
