I'd like to offer the following to this thread as it appears related. I recently had a very similiar issue with Gui Composer not starting in CCv5. After checking my network, disabling the firewall, disabling virus scanner, verifying JRE, Gui Composer would still not start. This is on a Windows 7 64-bit installation on a Core i5 system.
I use both VirtualBox and Vmware virtual machines. Both of these VMs install virtual network adapters to support the Host OS ethernet connections in the Guest OS. For some reason, if I disable all of these virtual network adapters, Gui Composer will start.
It may not be the same root cause as the above issues, but it seems worth adding to the list of items to verify when Gui Composer will not start.