You can try to start Windows Live Messenger with the /background parameter. But that parameter will only work if you also have enabled the option "
Open main window when Messenger starts". For more detailed info see:
CookieRevised's reply to Problem with WLM2011 when booting up computer
This said, I don't know if the behaviour of that parameter in combination with the startup options of Windows Live Messenger, is the same under Windows 7 as it is in Windows XP though. Especially since the taskbar and system tray in Windows 7 behave much differently than in Windows XP.
/ 
