Annoying! How Annoying! This very simple, very typical error message, but it is so hard to find a quick solution. Symptoms: You are starting a Cisco VPN Client and immediately you are getting this error message: Error 51: Unable to communicate with the VPN subsystem. Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again. Most of Internet discussions on this error message revolve around MAC OS. But this error I was able to fix on Windows XP. Quickly – What to Do? Quickly – Open your firewall. Each firewall solution is different, but…