Shoutbox

There is a script that can do this (which may or may not work on the latest version of Messenger)

[Release] Log Manager - Version 1.1.008

Or you could use something like DropBox to upload your chatlogs online, therefore you won't have to deal with them on multiple computers as they'll be in one central location which can be edited by any computer you use Messenger & Messenger Plus! Live on (as long as you set the chatlog location to be the same)