Why does Microsoft Office 2011 open automatically upon start up

I can't speak for Office 2011 because it requires you to activate your serial number with Microsoft, but in Office 2008, Office 2004 and Offive v.X, the incoming network connection is part of Microsoft's anti-piracy efforts. Since each copy of Office is activated with a unique product key, Office products open an incoming network connection and listen for connections from other instances of Microsoft Office on the network. When connections are established, the two instances of Office will trade product keys and make sure they are not the same. If they are the same, then one user is forced to close their copy of Office.

I recommend instructing your firewall NOT to allow Office to accept inbound network connections.

Here's a link to Macworld that speaks about this 'feature':

http://hints.macworld.com/article.php?story=20020406142423494

Quit all Office applications, and move com.microsoft.autoupdate and com.microsoft.office.plist from ~/Library/Preferences ... and any MicrosoftRegistrationDB .plists & the MicrosoftRegistrationDB.plist.lockfile from ~/Library/Preferences/ByHost to the desktop.

You may have to input your password for some of this. Start up an Office app again and see if this helps. The applications rebuild their preferences files. You may have to enter your registration code again.