It’s a typical BAD_EXEC. This could be related to two main things:
A bug in the program. The Mac programming framework (Called Cocoa) and the programming language used to develop most Mac programs (Objective-C) are very tricky with memory handling. A simple mistake could lead to unpredictable and unexpected crashes that seem to follow no “pattern”. It’s very common, it happens in the best software shops and until the vendor fixes the memory leak, there is nothing you can do (other than send them the bug report and pray).
If this behavior is exhibited by more software, there might be a problem with your hardware memory (RAM). Faulty RAM sticks are know to have caused this type of errors in a more or less unpredictable way. Programs start crashing and with time, new ‘problems’ arise, until the machine might eventually become almost unusable.
What can you do?
A simple test may be conducted by creating a new blank user in your computer, restarting your computer and using Messenger from the new user. This will rule any possible software problem with your user. If Messenger works fine under the new account, then further analysis of yours will lead us to try to find what’s interfering with MSN.
If it fails to work under the new account too, then the problem lies either in the MSN, Your OS X in general (weird but possible, usually a new user fixes 90% of the problems), or, worst case scenario, in your Hardware.
I suggest you try to above and take an action course. As a last resort, don’t hesitate taking the machine to an Apple Store/Genius Bar for further analysis. They will be able to provide you with much more information that what we could from here.