The Trac application does not use IIS to serve the web site. You do not need to install IIS on the server for the Trac application. If the Trac application is installed on a server that also runs other web pages from IIS, then you will need to set up IIS and the Trac application to not interfere with each other. By default, all web server applications serve on port 80. The Trac application may be modified to serve on a different port in order to allow IIS to serve on port 80.
To change the server port, open the file Prefs.INI that is located in the Trac application directory. You can use WordPad or Notepad to edit the Prefs.INI file. Locate ServerPort=80. Change 80 to 81 to serve the Trac application on port 81. Restart the Trac application and it will now be serving its web pages on port 81 (or another specified port).