I think TweakServers is correct -- your firewall/anti-virus software is likely blocking some ports. To access cPanel, for example, you need to have port 2082 (http) and 2083 (https) open. Your options:
1) Open the relevant ports.
2) If you are unable to open the needed ports, install cPanelProxy. It's great utility that enables you to use cPanel, WHM and Webmail through the normal port 80.