Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
MSN Messenger -> Preferences -> uncheck "Run This program when I start my computer."

arn
 
More generally, you can enable and disable apps to start when you login in System Preferences. Click on the Login Items pane, and select one app and click remove. Or to add something to your login items, click add and select the app you want to automatically launch.
 
Originally posted by benjaminpg
More generally, you can enable and disable apps to start when you login in System Preferences. Click on the Login Items pane, and select one app and click remove. Or to add something to your login items, click add and select the app you want to automatically launch.

You could do that, but i find that its better if you disable it within the program...I dont know if it matters, but I think it does.
 
It is better to disable it within the application itself. Virex is an example of this - you can remove Virex from System Preferences, but then next time you start Virex manually it'll go and add itself to your startup items again. You need to disable it within Virex itself to stop it from opening.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.