Check the Control Panel, Services applet and go to Properties of the PingPlotter Service. Go to the "Log On" tab and make sure that PingPlotter is set to Log on as: Local System account, and that the "Allow service to interact with the desktop" is turned On. These two settings are crucial (and PingPlotter automatically sets them when it's installed as a service, but maybe something didn't work right...).

Please let us know what you find there.