Installing an SSL Certificate for the Web Server
PRTG Network Monitor comes with a default SSL certificate so you can securely use the web interface through HTTPS out-of-the-box. But because it is not an official certificate that matches the domain name or IP address of your PRTG installation a web browser will always show an alert ("the certificate is not correct") when it connects to this server (although the transmission is already secure).
Please see the Paessler Knowledge Base for details:
http://www.paessler.com/support/kb/prtg7/tricks/install_ssl_certificate/