I thought I had this covered in one of my tips. Actually, I told you indirectly about how to uninstall MSN messenger and also how to stop from loading it at boot time. But recently I saw few mails/posts where people want to keep the MSN messenger , but do not want it to run whenever Outlook Express is executed.
For these folks here are a couple of solution. There is a Microsoft knowledgebase article on this as well. Visit How to Prevent Windows Messenger from Running on a Windows XP-Based Computer (Q302089) and learn more.
In regedit, navigate to [HKEY_LOCAL_MACHINE\\Software \\Policies \\Microsoft]. Create a new key by the name "Messenger" and then within that another key by the name "Client". This is equivalent to creating and navigation to [HKEY_LOCAL_MACHINE\\Software \\Policies\\Microsoft\\Messenger\\Client]. Within this create a DWORD key by the name "PreventRun" and set its value to 1. Quit registry and even though not a must, its a good idea to reboot. One more way of doing the same without sacrificing run time is to go to C:\\Program Files and rename Messenger directory to something else e.g. MessengerOff. This works as well.