Office Communicator 2007 has a feature that automatically opens the contacts list when communicator starts. But you can also disable auto open of contact list using Office Communicator 2007 options menu.
Click the down arrow below the Communicator Status icon and select Options from the list.
In the Option Personal Screen, navigate to My account section and un tick the checkbox with label as Automatically open the contact list when communicator starts.
Click OK button available at the bottom of the screen to confirm and save the changes.