Remember, this only works if you have Outlook XP (not Outlook Express). If you are missing the registry key below, this tip will not work for you. Always remember to backup your registry before you mess with it.
1. Open Regedit.
2. Go to HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Preferences
3. Add a new DWORD value and name it MinToTray
4. Set it equal to 1.
5. Restart Outlook, it should now minimize to the tray.
The above instructions are provided by TechTV