If you want to disable messenger, then HD's suggestion to rename the folder to anything other than its current name will work fine. If you want to remove messenger completely, then you must follow JOsh's suggestion. Other than a crowbar, that's the only way to remove it from your system.
Please note - you can always reinstall Messenger at a later date if you choose.
It's possible that by removing or disabling Windows Messenger, Outlook Express will launch several seconds slower.
I messed around with such configurations a while back to get messenger to stop launching -- things like uninstalling it from "add/remove programs" and disabling it in Outlook Express. However, one of the methods slowed down Outlook Express; unfortunately I don't know for sure which one it was.
The method I finally settled on was renaming the folder to "Messenger2" Now I have no extra slow down when launching Outlook Express.