Ok. That wouldn't help in this case though as this person wants to see the IM logs of other users. So the folder would need to be unencrypted first for the logs to be put there by the Messenger user thus it wouldn't help.
The OP would need some software that can run as a service to read the IMs of a limited user to be sure the logging couldn't be bypassed.
From a quick Google search there appears to be quite a bit of software designed to monitor IM messages.
Like parentsonpatrol.org which clams to silently record messages from MSN, Yahoo, AIM and MySpace IM and put them on it's web site for a parent to view when they want.
It apparently has a free version that has limits on the amount recorded and how long it's stored for.
I found that software by Google Search for "Recording Children's IM Conversations" then http://tech.yahoo.com/blog/raskin/546
Of which I would say like the yahoo page says one should tell the children that their IMs are being recorded.
Edit: Also with a service like that you would have to trust them with keeping the recorded IM message.