Plus! user some very advanced DLL Injection and Hooking method's. the Chat window is a single control. making it harder to get the messages inside of it. but not impossible.
what you'll have to do is to successfully inject your DLL into MSN's application. then hook the main procedure (win proc) (using SetWindowLong). when you've successfully done that, you'll have to do some MSN Reversing. find out what code/number/anything that is recieved when messages are written to the box.
there is only one other method i can think of that can get text from a chat. but i lost that source a while back. and it wasn't as hard as the Injection.
Good Luck. sorry i couldn't help more