The installation package must have internet access. The application checks a certificate upon installation (crl.verisign.com) and then downloads (through HTTP) the installation files from client.plaxo.com.
It's possible to install the application silently (at least with administrator privileges) using "PlaxoInstallNT.EX -s". It installs to %programfiles%, download files from internet and registers components.
Upon starting Outlook Express or Outlook the registration box for Plaxo come's up (it's a free registration for using the basic application).