Note in Outlook 2003 it can be done by Right-clicking the Outlook icon in the notification area, and select "Hide When Minimized" in the shortcut menu. More Info MS KB833007
[HKEY_CURRENT_USER \Software \Microsoft \Office \10.0 \Outlook \Preferences]
MinToTray = 1 (Default = 0, Enabled = 1, Disabled = 0)
Note when having enabled minimize to tray, then it is possible to make Outlook automatically place it self in the systray when it is started. This is done by creating a shortcut to the Outlook icon, which starts Outlook in minimized state. More Info MS KB251340.