"The fastest and safest way to remove Windows Messenger:
1) Click Start button
2) Click Run
3) Copy and paste this command into the Run dialog
4) RunDll32 advpack.dll,LaunchINFSection %windir%\INF\msmsgs.inf,BLC.Remove
5) Reboot and Windows Messenger is removed cleanly"
I got a "cannot unregister" message, but it worked. Phew.