This error is 100% related to connectivity, and is usually caused by third party applications like firewalls or proxies that are blocking OneCare from downloading the required files for installation.
The usual suspects are third party firewalls, proxy servers (if you are in a "work" environment), parental control software or internet "monitoring" software.
Basically anything that can get in between OneCare (the application) and the network card.
It would probably be a good idea to contact support if you have already tried most of the easy stuff, they will probably have some advanced troubleshooting that will help.