Actually, you can select port for Sitebuilder during installation process. You can also change it manually in the register. You should change following parameter:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\SiteBuilder\parameters -> Port
Just run Start -> Run -> regedit and modify this value. After that you need to restart Sitebuilder service: stop_service.cmd and start_service.cmd from ..\Sitebuilder\docroot directory.
Please be sure that this port is not occupied already and don’t forget to make copy of this value just in case.