I've been trying to get rid of windows messenger for ever on my laptop and finally found the magic bullet.
For those of you using XP, go to start menu->run->type gpedit.msc
It's the policy editor and lets you tweak a TON of stuff you'd normally not have access to.
Oh, and pop, you're officially not allowed to use this :D
Or DL TweakXP. It works, too. :>
(http://livejournal.com/users/pretentiousgit)