Actually you can effectively block MSN and P2P or any other program that you want to block by using the layer 7 filtering (application layer), you can actually implement this transparently to end user by just plugging the filtering server as brige to your gateway (or make it as a gateway if you have time to spend if you got into trouble as you need to apply the rules of your existing gateway). You can try out untagle if you can’t afford for the appliance one.

Good luck!