Or, provided that you have Messenger Plus! Live version 4.50+, enter the properties of Windows Live Messenger's shortcut, from which you launch Messenger (right click>Properties), add /PlusEnabled=0 at the end of the path written in the Target field in the properties window. Example:
code:
"C:\Program Files\Windows Live\Messenger\msnmsgr.exe" /PlusEnabled=0
Note the space before /Plus...