launch Windows Messenger and Tools -> Options.. -> Preferences
uncheck the checkbox for "Run Windows Messenger when Windows starts
this should prevent Windows Messenger from starting when Windows starts... otherwise, you can disable it using any startup mananger and disable Windows messenger to run at startup...
to uninstall Windows Messenger... and is provided by DjLizard:
Start, Run...
%windir%\inf\sysoc.inf
Edit > Replace
Find what: msmsgs.inf,hide,7
Replace with: msmsgs.inf,,7
Click Replace All
Click Cancel
File > Save
Go to Add/Remove Programs > Add/Remove Windows Components > Uncheck all Windows Messenger boxes > Next
Windows Messenger is actually useful to have installed, believe it or not. As you can have Remote Assistance done to your computer should you ever need it.