Office Communicator by default starts during Windows logon. But you can disable this behavior using Office Communicator options.
Launch Office Communicator, Click the down arrow available below the Office Communicator icon. Navigate to Tools menu and then click Options menu.
In the Options window under Personal tab navigate to My account section. Then un mark the check box labelled as Automatically start Communicator when I log on to Windows.
Click OK button available at the bottom of the Office Communicator Options window to confirm and save the changes.